Low-emission hydrocarbon cracking system

Abstract

A low-emission hydrocarbon cracking system for converting a hydrocarbon feedstock into cracked effluent gas is disclosed. The low-emission hydrocarbon cracking system can include a cracking furnace having a convection section heated by flue gas and configured to perform process and non-process heating operations, and a radiant section within which the hydrocarbon feedstock can be cracked by high temperature heating. The low-emission hydrocarbon cracking system may minimize the non-process duty of the cracking furnace convection section to ensure that sufficient convection section duty capacity is available even if flue gas output is reduced through use of a high efficiency radiant section. The non-process duty of the convection section may be minimized in some cases by superheating high pressure steam partly or completely outside of the convection section of the cracking furnace, such as by using one or more steam effluent exchangers that utilize waste heat from the cracked effluent gas. In an embodiment, a heat exchanger and cracking furnace arrangement comprises a transfer line exchanger (TLE) positioned in a cracked effluent gas flow path from a (fired) steam cracking furnace and a steam effluent exchanger positioned in the cracked effluent gas flow path in series with the transfer line exchanger, wherein the transfer line exchanger comprises a boiler water input and an at least partly vaporized boiler water output, the transfer line exchanger configured to at least partly vaporize the boiler water using waste heat from the cracked effluent gas; and wherein the steam effluent exchanger comprises a high pressure steam input and a superheated high pressure steam output, the steam effluent exchanger configured to generate the superheated high pressure steam using waste heat from the cracked effluent gas.

Description

Technical Field

[0001] The present disclosure relates generally to a low-emission hydrocarbon cracking system, and more particularly although not necessarily exclusively, to a low-emission hydrocarbon cracking system in which superheating of high pressure steam can be accomplished at least partially outside of a convection section of a furnace of the hydrocarbon cracking system.Background

[0002] A cracking furnace may be used to crack (break down the molecules of) hydrocarbons. The cracking process can allow heavier hydrocarbons to be broken down into more useful lighter hydrocarbons. One type of cracking furnace is a steam cracking furnace, wherein a hydrocarbon feedstock may be introduced into the furnace, diluted with steam, and heated in a radiant section such as a firebox to a sufficiently high temperature to crack the hydrocarbon feedstock. The hydrocarbon feedstock can be converted into various useful products, including among others, ethylene and propylene.Summary

[0003] Example No. 1 is a hydrocarbon cracking system for converting a hydrocarbon feedstock into a cracked effluent gas. The hydrocarbon cracking system includes a furnace, which comprises a convection section having at least one hydrocarbon feedstock preheater, a radiant section comprising at least one radiant coil arranged to receive the preheated hydrocarbon feedstock and to generate the cracked effluent gas therefrom, and a cooling apparatus arranged to receive the cracked effluent gas. The cooling apparatus comprises a transfer line exchanger arranged to receive a boiler water flow and to generate at least partly vaporized boiler water therefrom using waste heat from the cracked effluent gas, and a steam effluent exchanger arranged to receive a high pressure steam flow and to generate superheated high pressure steam therefrom using waste heat from the cracked effluent gas.

[0004] Example No. 2 is Example No. 1, further comprising a high pressure steam source that receives the at least partly vaporized boiler water from the transfer line exchanger. The high pressure steam source is in fluid communication with the steam effluent exchanger.

[0005] Example No. 3 is Example No. 2, wherein the steam effluent exchanger is arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas. The steam effluent exchanger is located either downstream or upstream of the transfer line exchanger, and a flow path of the high pressure steam flow through the steam effluent exchanger is either counter-current to or co-current with the flow path of the cracked effluent gas.

[0006] Example No. 4 is Example No. 2, further comprising a high pressure steam superheater located in the convection section of the furnace and arranged to receive the superheated high pressure steam from the steam effluent exchanger and to further superheat the superheated high pressure steam.

[0007] Example No. 5 is Example No. 2, wherein the steam effluent exchanger comprises at least two steam effluent exchangers. A first steam effluent exchanger of the at least two steam effluent exchangers is arranged to receive the high pressure steam flow from the high pressure steam source and to generate the superheated high pressure steam therefrom. A second steam effluent exchanger of the at least two steam effluent exchangers is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam. A desuperheater is located in a flow path of the superheated high pressure steam between the first steam effluent exchanger and the second steam effluent exchanger and is operable to control a temperature of the superheated high pressure steam leaving the second steam effluent exchanger by injecting cold boiler feed water into the superheated high pressure steam.

[0008] Example No. 6 is Example No. 2, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger is located downstream of the transfer line exchanger and the second steam effluent exchanger is located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas. The second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

[0009] Example No. 7 is Example No. 2, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger is located downstream of the transfer line exchanger and the second steam effluent exchanger is located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is co-current with the flow path of the cracked effluent gas The second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

[0010] Example No. 8 is Example No. 2, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas. The first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas. The second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

[0011] Example No. 9 is Example No. 2, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas. The first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is co-current with the flow path of the cracked effluent gas. The second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

[0012] Example No. 1 0 is Example No. 2, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with a transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas. The first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas. The second steam effluent exchanger is configured to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is co-current with the flow path of the cracked effluent gas.

[0013] Example No. 11 is a heat exchanger and cracking furnace arrangement. The heat exchanger and cracking furnace arrangement includes a transfer line exchanger positioned in a cracked effluent gas flow path from a cracking furnace, and a steam effluent exchanger positioned in the cracked effluent gas flow path in series with the transfer line exchanger. The transfer line exchanger comprises a boiler water input and an at least partly vaporized boiler water output, and is configured to at least partly vaporize the boiler water using waste heat from the cracked effluent gas. The steam effluent exchanger comprises a high pressure steam input and a superheated high pressure steam output, and is configured to generate the superheated high pressure steam using waste heat from the cracked effluent gas.

[0014] Example No. 12 is Example No. 11, wherein the steam effluent exchanger comprises at least two steam effluent exchangers. A first steam effluent exchanger of the at least two steam effluent exchangers is arranged to receive a high pressure steam flow from a high pressure steam source and to generate the superheated high pressure steam therefrom. A second steam effluent exchanger of the at least two steam effluent exchangers is positioned to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and is configured to further superheat the superheated high pressure steam. The second steam effluent exchanger is configured to direct the flow of the superheated high pressure steam through the second steam effluent exchanger along a path that is co-current with or counter-current to a path of the high pressure steam flow through the first steam effluent exchanger.

[0015] Example No. 13 is Example No. 12, further comprising a desuperheater that is positioned in a flow path of the superheated high pressure steam between the first steam effluent exchanger and the second steam effluent exchanger and is operable to control a temperature of the superheated high pressure steam leaving the second steam effluent exchanger by injecting cold boiler feed water into the superheated high pressure steam.

[0016] Example No. 14 is a method of operating a hydrocarbon cracking system to convert a hydrocarbon feedstock into a cracked effluent gas. The method includes preheating the hydrocarbon feedstock in a convection section of a cracking furnace of the hydrocarbon cracking system, and generating a cracked effluent gas by heating the preheated hydrocarbon feedstock in a radiant section of the cracking furnace. The method also includes cooling the cracked effluent gas by passing the cracked effluent gas through a transfer line exchanger and generating at least partly vaporized boiler water by heating boiler water in the transfer line exchanger using waste heat from the cracked effluent gas. The method further includes generating high pressure steam, at least in part, from the at least partly vaporized boiler water, and generating superheated high pressure steam by heating a flow of the high pressure steam in a steam effluent exchanger using waste heat from the cracked effluent gas.

[0017] Example No. 15 is Example No. 14, wherein the high pressure steam is provided by a steam drum from a combination of the boiler water present in the steam drum and the at least partly vaporized boiler water generated in the transfer line exchanger, and the flow of the high pressure steam is received by the steam effluent exchanger from the steam drum.

[0018] Example No. 16 is Example No. 14, wherein the steam effluent exchanger is arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas, the steam effluent exchanger is located either downstream or upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the high pressure steam flows through the steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas.

[0019] Example No. 17 is Example No. 14, wherein the superheated high pressure steam generated in the steam effluent exchanger is further superheated by directing the superheated high pressure steam to a high pressure steam superheater located in the convection section of the cracking furnace.

[0020] Example No. 18 is Example No. 14, wherein the steam effluent exchanger comprises at least two steam effluent exchangers. A first steam effluent exchanger of the at least two steam effluent exchangers receives the flow of the high pressure steam from a high pressure steam source and generates the superheated high pressure steam therefrom. A second steam effluent exchanger of the at least two steam effluent exchangers receives a flow of the superheated high pressure steam from the first steam effluent exchanger and further superheats the superheated high pressure steam. A desuperheater is also located in a flow path of the superheated high pressure steam between the first steam effluent exchanger and the second steam effluent exchanger and is used to control a temperature of the superheated high pressure steam leaving the second steam effluent exchanger by injecting cold boiler feed water into the superheated high pressure steam.

[0021] Example No. 19 is Example No. 14, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger, and the first steam effluent exchanger and the second steam effluent exchanger are arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger is located downstream of the transfer line exchanger along the flow path of the cracked effluent gas and the second steam effluent exchanger is located upstream of the transfer line exchanger along the flow path of the cracked effluent gas. The superheated high pressure steam is generated in the first steam effluent exchanger by directing the flow of the high pressure steam through the first steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas. The superheated high pressure steam generated in the first steam effluent exchanger is further superheated in the second steam effluent exchanger by directing a flow of the superheated high pressure steam through the second steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas.

[0022] Example No. 20 is Example No. 14, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger, and the first steam effluent exchanger and the second steam effluent exchanger are arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas. The superheated high pressure steam is generated in the first steam effluent exchanger by directing the flow of the high pressure steam through the first steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas. The superheated high pressure steam generated in the first steam effluent exchanger is further superheated in the second steam effluent exchanger by directing a flow of the superheated high pressure steam through the second steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas.Brief Description of the Drawings

[0023] FIG. 1 is a schematic representation of a low-emission hydrocarbon cracking system according to one example configuration. FIG. 2 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 3 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 4 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 5 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 6 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 7 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 8 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 9 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 10 is a schematic representation of a portion of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 11 is a schematic representation of a portion of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 12 is a schematic representation of a portion of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 13 is a flowchart describing one example of a method of cracking a hydrocarbon feedstock using a low-emission hydrocarbon cracking system. Detailed Description

[0024] Certain aspects and examples of the present disclosure relate to a low-emission hydrocarbon cracking system that includes a cracking furnace having a convection section and a radiant section and is operable to crack a hydrocarbon feedstock. The low-emission hydrocarbon cracking system may achieve lower emissions by, for example, facilitating the ability to increase the efficiency of the radiant section (e.g., firebox) of the cracking furnace without exceeding a duty capacity of a convection section of the cracking furnace. This may be achieved in some examples by utilizing configurations where the process of superheating high pressure steam used for power generation or other downstream processes is performed partly or completely outside the convection section of the cracking furnace. Superheating the high pressure steam outside the convection section lessens the heat demand of the convection section on the flue gas that is generated during the hydrocarbon cracking process. This reduction in convection section heat demand can ensure that the convection section still has an adequate duty capacity even if the amount of heat that is providable thereto by the flue gas is reduced as a result of increasing the efficiency of the radiant section.

[0025] A low-emission hydrocarbon cracking system according to an example configuration may generally include a convection section, in which hydrocarbon feedstock can be preheated and / or partly evaporated and mixed with dilution steam to provide a feedstock-dilution steam mixture. The system can also comprise a radiant section, which may include, in some examples, at least one radiant coil in a firebox, and in which the feedstock-dilution steam mixture from the convection section can be converted into product and byproduct components at high temperature by pyrolysis. A hydrocarbon cracking system example can further comprise a cooling apparatus including at least one transfer line exchanger, that may be configured to quickly quench the product or cracked effluent gas leaving the radiant section of the cracking furnace in order to stop pyrolysis side reactions, and to preserve the equilibrium of the reactions in favor of the products. Heat from the transfer line exchanger can be recovered in the form of the high pressure steam.

[0026] In an example configuration of a low-emission hydrocarbon cracking system, the convection section may include multiple convection banks for preheating the feedstock and for other purposes, such as for example, heating boiler water, heating a mixture of the feedstock and dilution steam, superheating the dilution steam, and superheating the high pressure steam. The convection section banks receive heat from flue gas that is generated in the radiant section and flows through the convection section. Each of the operations performed by the convection banks of the convection section are part of the duty of the convection section and the convection section needs to have sufficient duty capacity to perform the operations.

[0027] In some examples, the required non-process duty of the convection section can be minimized by performing in another manner, one or more operations typically performed in the convection section. For example, the non-process duty of the convection section can be minimized by superheating high pressure steam outside the convection section using non-convection section components. Removing part or all of the high pressure steam superheating responsibility from the convection section allows the convection section duty to be minimized without affecting the process duty, which includes the duty related to preheating or preheating and vaporizing the feedstock, heating the feedstock-dilution steam mixture, and superheating the dilution steam. With the convection section non-process duty minimized, the radiant section (e.g., fire box) efficiency can be increased without concern for high pressure steam superheating limitations of the convection section that could otherwise negatively impact the quality of high pressure steam that is available for effective downstream power generation. For example, and as described in more detail below, insufficiently superheated high pressure steam can cause condensation problems in downstream turbines to which the superheated high pressure steam is directed.

[0028] Example configurations of a low-emission hydrocarbon cracking system may use the heat of the effluent produced by the pyrolysis process (i.e., the cracked effluent gas) to superheat the high pressure steam instead of using heat from the flue gas. For example, one or more steam effluent exchangers may be placed in a flow path of the cracked effluent gas that is discharged from the radiant section of the cracking furnace. High pressure steam, such as from a steam drum or another high pressure steam source, may be directed through the one or more steam effluent exchangers to superheat the high pressure steam using the waste heat of the cracked effluent gas that also flows therethrough.

[0029] In some examples, only one steam effluent exchanger may be used, and the steam effluent exchanger may be located upstream or downstream of the transfer line exchanger used to cool the cracked effluent gas stream.

[0030] In some examples, the superheated high pressure steam may be discharged from the steam effluent exchanger for downstream use. In other examples, superheating of the high pressure steam may be a two-stage process, where the steam effluent exchanger is used in a first stage high pressure steam superheating operation. In such an example, a second stage high pressure steam superheating operation may be performed by directing the initially superheated high pressure steam from the steam effluent exchanger to a high pressure steam superheater in the convection section.

[0031] In some examples, more than one steam effluent exchanger may be used to superheat the high pressure steam. In such an example, the steam effluent exchangers may be placed in different arrangements relative to the transfer line exchanger. For example, when two steam effluent exchangers are used, one steam effluent exchanger may be located upstream of the transfer line exchanger and the other steam effluent exchanger may be located downstream of the transfer line exchanger. In another example configuration in which two steam effluent exchangers are used, both of the steam effluent exchangers may be located upstream of the transfer line exchanger.

[0032] In some examples, the flow of high pressure steam through a steam effluent exchanger may be co-current with the flow of the cracked effluent gas through the steam effluent exchanger. In other examples, the flow of high pressure steam through a steam effluent exchanger may be counter-current to the flow of the cracked effluent gas through the steam effluent exchanger. In an example where more than one steam effluent exchanger is used to superheat high pressure steam, the flow of high pressure steam through each steam effluent exchanger may be co-current with the flow of the cracked effluent gas therethrough, counter-current to the flow of the cracked effluent gas therethrough, or may be co-current with respect to one or some of the steam effluent exchangers and counter-current to one or some of the other steam effluent exchangers.

[0033] When more than one steam effluent exchanger is used, the arrangement of the steam effluent exchangers relative to each other can reduce the required surface area of one or some of the steam effluent exchangers. The arrangement of the steam effluent exchangers relative to each other may also allow for the use of previously unusable steam effluent exchanger materials by reducing the maximum tube wall temperature that will be experienced by a steam effluent exchanger.

[0034] In some examples, the pressure of the high pressure steam generated by a low-emission hydrocarbon cracking system may be, for example, between 15 bar and 130 bar. In some examples, the temperature of the superheated high pressure steam generated by the one or more steam effluent exchangers may vary depending on the intended downstream use of the steam. For example, if the downstream steam system contains one or more condensing turbines, the temperature of the superheated high pressure steam can be controlled so that the resulting superheated high pressure steam does not produce condensation in the condensing turbines that exceeds a maximum allowable condensation percentage. In another example, where the downstream steam system includes only a back pressure turbine(s), the temperature of the superheated high pressure steam can be controlled so that the resulting superheated high pressure steam does not produce condensation in the back pressure turbine(s). Generally speaking, the high pressure steam will typically require less superheating when it will be directed to a back pressure turbine(s) than when it will be directed to a condensing turbine(s). In another example, where the high pressure steam is used as heating medium, there may be no such restrictions, but the temperature of the superheated high pressure steam should nonetheless be sufficient to prevent steam condensation in steam supply lines located upstream of heat exchangers to which the high pressure steam is directed. In some examples, the temperature of the superheated high pressure steam can be between 200 °C to 550 °C, depending on the pressure of the high pressure steam and the amount of superheating required to prevent turbine condensation damage. In one example, the temperature of the superheated high pressure steam may be about 400 °C.

[0035] Illustrative examples follow and are given to introduce the reader to the general subject matter discussed herein rather than to limit the scope of the disclosed concepts. The following sections describe various additional features and examples with reference to the drawings in which like numerals indicate like elements, and directional descriptions are used to describe the illustrative aspects, but, like the illustrative aspects, should not be used to limit the present disclosure.

[0036] FIG. 1 is a schematic representation of one example configuration of a low-emission hydrocarbon cracking system 100 in which high pressure steam 4 is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 101 of the low-emission hydrocarbon cracking system 100. As shown, during operation of the low-emission hydrocarbon cracking system 100, a hydrocarbon feedstock 1 can be introduced to a feed preheater 22, which may be one convection bank 21 of multiple convection banks 21 in a convection section 20 of the cracking furnace 101. The hydrocarbon feedstock 1 can be one of various usable hydrocarbons, and may be paraffinic or naphthenic in nature. Small quantities of aromatics and olefins can also be present. For example, the hydrocarbon feedstock may be ethane, propane, butane, natural gasoline, naphtha, kerosene, natural gas condensate, gas oil, vacuum gas oil, hydrotreated or desulphurized or hydrodesulphurized (vacuum) gas oils, or combinations thereof.

[0037] The convection section 20 of the cracking furnace 101 may more specifically include, from top to bottom in this example, a stack 31 that employs a flue gas fan 30 to evacuate flue gas 7, and multiple convection banks 21 that include: an air preheater 27 that is usable to preheat combustion air; the feed preheater 22; a first high temperature coil 23 that is usable to heat a mixture of the hydrocarbon feedstock 1 and dilution steam 2; a dilution steam superheater 24 that is usable to superheat the dilution steam 2; a high pressure steam superheater 25 that is usable to superheat saturated high pressure steam; and a second high temperature coil 26 that is usable to heat the mixture of the hydrocarbon feedstock 1 and dilution steam 2. The type, number, and arrangement of the convection banks 21 may be different in other cracking furnace examples.

[0038] The hydrocarbon feedstock 1 may be preheated or partially or fully evaporated in the feed preheater 22 before being mixed with the dilution steam 2. The dilution steam 2 can be injected directly, or may be superheated before being mixed with the hydrocarbon feedstock 1. In this example, the dilution steam 2 can be superheated in the dilution steam superheater 24 prior to being mixed with the hydrocarbon feedstock 1. In some cases, multiple steam injection points may be provided to help dilute heavier hydrocarbon feedstocks.

[0039] A mixture of the hydrocarbon feedstock 1 and the dilution steam 2 can be further heated by the high temperature coils 23, 26 to reach an optimum temperature prior to introduction to a radiant section (i.e., firebox) 10. Within the firebox 10 is a radiant coil 11 within which the hydrocarbon feedstock 1 travels and is quickly heated to a temperature at which a pyrolysis reaction occurs. The heat of reaction for the pyrolysis reaction, which is highly endothermic, can be supplied by the combustion of fuel (e.g., gas) 5 with combustion air 6 in the firebox 10. Combustion can be performed by bottom burners 12, side wall burners (not shown), or a combination thereof. The radiant coil 11 may run a length of the firebox 10 to distribute the heat as evenly as possible to the radiant coil and the hydrocarbon feedstock located therein. The efficiency of this example of the cracking furnace 101 is determined by the heat release pattern of the bottom burners 12 and the extent of the air preheating.

[0040] Combustion air 6 can be provided by a forced draft combustion air fan 37, which can force the combustion air 6 through an air preheater 27 situated in the convection section 20 where the combustion air 6 can recover waste heat from the flue gas 7. The preheated combustion air 6 can then be sent to the bottom burners 12 of the firebox 10 to combust the fuel 5. This produces a hot combustion zone 14. The efficiency of the firebox 10 can be improved by locating the combustion zone 14 lower in the firebox 10 and increasing the combustion zone temperature. The bottom burners 12 provide heat low in the firebox 10 and preheating of the combustion air 6 increases the temperature of the combustion zone 14. Further, liberating more flue gas lower in the firebox 10 makes the flue gas flow more co-current compared to liberating the flue gas more evenly throughout the firebox 10, and also results in a greater interaction between the hot flue gas and the radiant coil 11. The hotter combustion temperature raises the temperature driving force, especially improving the radiative heat transfer. Both the restriction of the combustion zone 14 to the lower part of the firebox 10 and the preheating of the combustion air can be used to improve firebox efficiency, thus reducing fuel consumption and associated CO 2 emissions. In another example (not shown), a similar result may be obtained by locating multiple rows of side wall burners with air preheat in a lower portion of the vertical walls of the firebox 10 in lieu of the bottom burners 12.

[0041] Through the pyrolysis reaction, the hydrocarbon feedstock is converted into products and by-products. Examples of such products can include hydrogen, ethylene, propylene, butadiene, benzene, toluene, styrene and xylenes. Byproducts may include methane and fuel oil. The resulting mixture of the dilution steam 2, unconverted hydrocarbon feedstock 1, and converted feedstock, creates a reactor effluent that may be referred to as "cracked effluent gas" 8. The cracked effluent gas 8 can be cooled in a cooling apparatus 102 of the low-emission hydrocarbon cracking system 100 in an effort to maintain the equilibrium of the pyrolysis reactions in favor of the products rather than the byproducts.

[0042] At least some of the waste heat in the cracked effluent gas can be recovered in a transfer line exchanger 32 of the cooling apparatus 102 of the low-emission hydrocarbon cracking system 100 and may be used to generate saturated high pressure steam 4. The high pressure steam 4 can be generated, for example, using boiler water 9a from a steam drum 33. Boiler feed water 3 may be provided to the steam drum 33 for this purpose. The boiler feed water 3 can mix with boiler water 9 already present in the steam drum 33. The boiler water 9a from a steam drum 33 is partly vaporized in the transfer line exchanger 32. The partly vaporized boiler water 9b can flow back to the steam drum 33 by way of natural circulation. Additionally, the high pressure steam 4 can be separated from the boiler water in the steam drum 33. To control the high pressure steam temperature, boiler feed water may be injected in a desuperheater 34.

[0043] Combustion of the fuel 5 with the combustion air 6 in the radiant section 10 of the cracking furnace 101, generates combustion products such as water and CO 2 , which is commonly referred to as "flue gas" 7. Waste heat from the flue gas 7 may be recovered in the convection section 20 using various types of the convection banks 21. Some of the waste heat can be used for process side purposes, such as for preheating and / or evaporating the hydrocarbon feedstock 1 and the dilution steam 2. The remainder of the waste heat may be used for the non-process side purposes, such as to generate the high pressure steam 4.

[0044] It is possible in some cases to superheat the high pressure steam using one or more convection banks in the convection section 20 of the furnace 101 using the flue gas 7 as the heating medium. For example, when sufficient heat is generated in the firebox 10, enough of the heat may be recoverable in the convection section 20 to provide the convection section 20 with the available convection duty required to perform process convection duties and to also superheat the generated high pressure steam 4. However, if the firebox efficiency is increased, which is a desirable improvement, at least the non-process duty of the convection section 20 will be significantly reduced and there may not be sufficient heat in the flue gas 7 to allow the convection section 20 to fulfill its process duty functions and also superheat the high pressure steam 4.

[0045] The low-emission hydrocarbon cracking system 100, as well as other low-emission hydrocarbon system examples according to the disclosure, can thus be configured to allow for additional improvements in radiant section efficiency while still ensuring sufficient process and non-process duty capacity in the convection section 20. In the low-emission hydrocarbon cracking system 100 configuration of FIG. 1 (and in the other low-emission hydrocarbon cracking system examples that follow), a non-process duty of the convection section 20 of the cracking furnace 101 can be minimized by removing at least a portion of the high pressure steam superheating burden therefrom. To this end, the cooling apparatus 102 of the low-emission hydrocarbon cracking system 100 can further include a at least one steam effluent exchanger 60 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 101 can be further cooled, and by which superheated high pressure steam can be generated using waste heat from the cracked effluent gas 8.

[0046] In the case of the low-emission hydrocarbon cracking system 100, the cracked effluent gas 8 generated in the radiant section 10 of the cracking furnace 101 can be initially cooled in the transfer line exchanger 32 and then further cooled in the steam effluent exchanger 60 that is downstream of the transfer line exchanger 32 relative to the flow path of the cracked effluent gas 8 upon its exit from the radiant section 10. Boiler feed water 3 can be sent to a steam drum 33, and boiler water 9a from the steam drum 33 may be directed to the transfer line exchanger 32. The boiler water 9a can be at least partly vaporized in the transfer line exchanger 32 by heat recovered from the stream of cracked effluent gas 8 passing through the transfer line exchanger 32. The partly vaporized boiler water 9b can then flow back to the steam drum 33 by natural circulation. In the steam drum 33, water can be separated from high pressure steam 4 and the high pressure steam 4 can be sent to the steam effluent exchanger 60 where the high pressure steam 4 can be superheated by heat recovered from the stream of cracked effluent gas 8 passing through the steam effluent exchanger 60. As shown, the flow of the high pressure steam 4 through the steam effluent exchanger 60 may be counter-current to the flow of cracked effluent gas 8 in this example.

[0047] As may be understood from FIG. 1 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 101 of the low-emission hydrocarbon cracking system 100 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 101. This helps to minimize the non-process duty load of the convection section 20 of the cracking furnace 101.

[0048] In the low-emission hydrocarbon cracking system 100, superheating of the high pressure steam 4 in the steam effluent exchanger 60 may be a first stage of high pressure steam superheating. In this example, a second stage of high pressure steam superheating can still be performed by directing the high pressure steam 4 discharged from the steam effluent exchanger 60 to the high pressure steam superheater 25 in the convection section 20 of the cracking furnace 101 to be further superheated using heat from the flue gas 7 that travels from the radiant section 10 through the convection section 20. To control the temperature of the high pressure steam 4 directed to the high pressure steam superheater 25 from the steam effluent exchanger 60, the high pressure steam may be passed through a desuperheater 34 prior to its introduction to the high pressure steam superheater 25. The desuperheater 34 may operate to cool the high pressure steam by injecting cold boiler feed water into the high pressure steam 4. Use of the desuperheater 34 in this manner can allow for precise control over the superheating of the high pressure steam 4 leaving the high pressure steam superheater 25. This can be beneficial to power generation, especially when the downstream steam system utilizes condensing turbines, as it generally does in ethylene plants.

[0049] A reduction in fuel consumption and a corresponding decrease in CO 2 emissions associated with the low-emission hydrocarbon cracking system 100 and the other low-emission hydrocarbon cracking system examples that follow can be facilitated as described in detail above - by improving firebox efficiency through restriction of the combustion zone 14 to the lower part of the firebox 10 and preheating of the combustion air 6. When fuel consumption is reduced (i.e., less fuel is burned), flue gas production is also reduced. The production of less flue gas 7 results in a reduced convection duty of the convection section 20. If the resulting reduced convection duty of the convection section 20 becomes insufficient to satisfy the required process and non-process duty requirements associated with operation of the cracking furnace 101, then the convection duty of the convection section needs to be reduced in order to ensure proper cracking furnace operation.

[0050] One way to reduce the convection duty of the convection section 20 is to utilize a feed effluent exchanger that uses a mixture of feedstock and dilution steam on the shell side of the exchanger instead of high pressure steam, which can reduce the convection section process duty heat requirements. However, the feedstock itself can be subject to fouling reactions and the feedstock-dilution steam mixture must enter the shell side of the exchanger in the vapor phase to prevent fouling by contact between liquid droplets and the hot tube surface. As some feedstocks can be relatively heavy and can have a boiling range with a high end point, it can be a challenge to fully evaporate these feedstocks at a temperature that is low enough to make the feed effluent exchanger an effective tool for removing sufficient convection duty from the convection section.

[0051] Alternatively, and as described in further detail below, the non-process duty heat requirements of the convection section 20 can be reduced by utilizing one or more steam effluent exchangers that can be arranged to receive a high pressure steam flow and configured to generate superheated high pressure steam from the high pressure steam flow using waste heat from the cracked effluent gas produced by the cracking furnace 101. This can remove or at least reduce the high pressure steam superheating requirements of the convection section 20, thereby allowing the cracking furnace 101 to operate properly even when flue gas generation is decreased as a result of reduced fuel consumption. Further, with saturated high pressure steam on the shell side of the steam effluent exchanger, sufficient throughput on both sides of the exchanger can be ensured and shell-side fouling can be avoided. This can also permit a low-emission hydrocarbon cracking system according to the present disclosure to utilize a broad range of hydrocarbon feedstocks without shell-side fouling concerns.

[0052] In the drawing figures associated with the following low-emission hydrocarbon cracking system configuration examples of FIGS. 2-9, at least redundant portions of the cracking furnace 101 of FIG. 1 have been reused for clarity and to better illustrate differences between the various low-emission hydrocarbon cracking system configuration examples presented herein. Components of the low-emission hydrocarbon cracking system configuration examples illustrated in FIGS. 2-9 that have commonality with the components of the cracking furnace 101 of FIG. 1 are indicated by the same reference numbers used in FIG. 1.

[0053] FIG. 2 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 200 in which high pressure steam is superheated at least in part by a mechanism other than a convection section of a cracking furnace 201 of the low-emission hydrocarbon cracking system 200. The cracking furnace 201 of the low-emission cracking furnace 200 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 200 is also shown to include a cooling apparatus 202 including a transfer line exchanger 32 and a steam effluent exchanger 60 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 201 can be cooled and by which superheated high pressure steam can be generated. Operation of the low-emission hydrocarbon cracking system 200 is substantially the same as described relative to the low-emission hydrocarbon cracking system 100, except that the flow of high pressure steam 4 through the steam effluent exchanger 60 of the cooling apparatus 202 of the low-emission hydrocarbon cracking system 200 is co-current to the flow of cracked effluent gas 8 through the steam effluent exchanger 60 in this example instead of counter-current as in the low-emission hydrocarbon cracking system 100.

[0054] As may be understood from FIG. 2 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 201 of the low-emission hydrocarbon cracking system 200 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 201. This again helps to minimize the non-process duty load of the convection section 20.

[0055] FIG. 3 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 300 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 301 of the low-emission hydrocarbon cracking system 300. The cracking furnace 301 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 300 is also shown to include a cooling apparatus 302 including a transfer line exchanger 32 and a steam effluent exchanger 60 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 301 can be cooled and by which superheated high pressure steam can be generated.

[0056] In the low-emission hydrocarbon cracking system 300, the heat of the cracked effluent gas 8 stream can initially be used to superheat high pressure steam 4 in the steam effluent exchanger 60 prior to the cracked effluent gas 8 being further cooled in the transfer line exchanger 32 that is in series with and downstream of the steam effluent exchanger 60 along the flow path of the cracked effluent gas 8. Boiler feed water 3 can be sent to a steam drum 33 and boiler water 9a from the steam drum 33 may be directed to the transfer line exchanger 32. The boiler water 9a can be at least partly vaporized in the transfer line exchanger 32 by heat recovered from the stream of cracked effluent gas 8 passing therethrough. The partly vaporized boiler water 9b can then flow back to the steam drum 33 by natural circulation. In the steam drum 33, water can be separated from high pressure steam 4 and the high pressure steam 4 can be sent to the steam effluent exchanger 60 for superheating of the high pressure steam 4 in steam effluent exchanger 60 in the manner previously described with respect to the low-emission hydrocarbon cracking system 100 of FIG. 1. As shown, the flow of the high pressure steam 4 through the steam effluent exchanger 60 may again be counter-current to the flow of cracked effluent gas 8 therethrough.

[0057] As may be understood from FIG. 3 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 301 of the low-emission hydrocarbon cracking system 300 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 301, and resultantly helps to minimize the non-process duty load of the convection section 20 of the cracking furnace 301. Likewise, superheating of the high pressure steam 4 in the steam effluent exchanger 60 may again be a first stage of high pressure steam superheating, and the superheated high pressure steam may be passed to the high pressure steam superheater 25 in the convection section 20 of the cracking furnace 301 for further (second stage) superheating as described above with respect to the low-emission hydrocarbon cracking system 100 of FIG. 1. A desuperheater 34 may be used to control the temperature of the superheated high pressure steam leaving the high pressure steam superheater 25.

[0058] FIG. 4 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 400 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 401 of the low-emission hydrocarbon cracking system 400. The cracking furnace 401 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 400 is also shown to include a cooling apparatus 402 including a transfer line exchanger 32 and a steam effluent exchanger 60 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 401 can be cooled and by which superheated high pressure steam can be generated. Operation of the low-emission hydrocarbon cracking system 400 is substantially similar to the operation of the low-emission hydrocarbon cracking system 300 of FIG. 3, as the low-emission hydrocarbon cracking system 400 also has a steam effluent exchanger 60 arranged upstream of a transfer line exchanger 32. As may be observed, the flow of the high pressure steam 4 through the steam effluent exchanger 60 is co-current to the flow of cracked effluent gas 8 therethrough in this example instead of counter-current as in the low-emission hydrocarbon cracking system 300.

[0059] As may be understood from FIG. 4 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 401 of the low-emission hydrocarbon cracking system 400 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 401. The non-process duty load of the convection section 20 of the cracking furnace 401 may thus again be minimized.

[0060] FIG. 5 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 500 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 501 the low-emission hydrocarbon cracking system 500. The cracking furnace 501 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 500 is also shown to include a cooling apparatus 502 including a transfer line exchanger 32 and a first and a second steam effluent exchanger 61, 62 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 501 can be cooled and by which superheated high pressure steam can be generated.

[0061] In the case of the low-emission hydrocarbon cracking system 500, the cracked effluent gas 8 may pass through each of the second steam effluent exchanger 62, a transfer line exchanger 32 that is downstream of the second steam effluent exchanger 62, and the first steam effluent exchanger 61 that is downstream of the transfer line exchanger 32. As in previous examples, boiler feed water 3 can be sent to a steam drum 33, and boiler water 9a from the steam drum 33 may be directed to the transfer line exchanger 32. The boiler water 9a can be at least partly vaporized in the transfer line exchanger 32 by heat recovered from the stream of cracked effluent gas 8 passing therethrough, and the partly vaporized boiler water 9b can then flow back to the steam drum 33 by natural circulation. In the steam drum 33, water can be separated from high pressure steam 4 and the high pressure steam 4 can be sent to the first steam effluent exchanger 61 where the high pressure steam 4 can be initially superheated in a first stage superheating process using heat recovered from the stream of cracked effluent gas 8 passing through the first steam effluent exchanger 61. The high pressure steam 4 initially superheated in the first stage superheating process can then be subjected to second stage superheating by directing the superheated high pressure steam 4 from the first steam effluent exchanger 61 to the second steam effluent exchanger 62 and using the heat in the stream of cracked effluent gas 8 passing through the second steam effluent exchanger 62 to further superheat the superheated high pressure steam 4.

[0062] The flow of the high pressure steam 4 through the first steam effluent exchanger 61 and the second steam effluent exchanger 62 is counter-current to the flow of cracked effluent gas 8 in this example. The temperature of the superheated high pressure steam 4 leaving the second steam effluent exchanger 62 may be controlled by passing the superheated high pressure steam through a desuperheater 34 prior to its introduction to the second steam effluent exchanger 62. The desuperheater 34 may operate as described above for this purpose.

[0063] As may be understood from FIG. 5 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 501 of the low-emission hydrocarbon cracking system 500 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 501. Thus, the non-process duty load of the convection section 20 of the cracking furnace 501 is again minimized, and is also further reduced relative to the previous low-emission hydrocarbon cracking system examples presented herein, as use of the second steam effluent exchanger 62 for second stage superheating of the high pressure steam 4 eliminates the need to provide the convection section 20 of the cracking furnace 501 with the high pressure steam superheater 25 of the previous examples.

[0064] FIG. 6 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 600 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 601 the low-emission hydrocarbon cracking system 600. The cracking furnace 601 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 600 is also shown to include a cooling apparatus 602 including a transfer line exchanger 32 and a first and a second steam effluent exchanger 61, 62 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 601 can be cooled and by which superheated high pressure steam can be generated.

[0065] Operation of the low-emission hydrocarbon cracking system 600 is substantially the same as described relative to the low-emission hydrocarbon cracking system 500, except that the flow of the high pressure steam 4 through the first steam effluent exchanger 61 is co-current to the flow of cracked effluent gas 8 through the first steam effluent exchanger 61, while the flow of the superheated high pressure steam 4 through the second steam effluent exchanger 62 remains counter-current to the flow of cracked effluent gas 8 therethrough as in the low-emission hydrocarbon cracking system 500. A desuperheater 34 may again be used to control the temperature of the superheated high pressure steam 4 leaving the second steam effluent exchanger 62.

[0066] As may be understood from FIG. 6 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 601 of the low-emission hydrocarbon cracking system 600 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 601. Thus, the non-process duty load of the convection section 20 of the cracking furnace 601 is again minimized, and is also further reduced relative to the previous low-emission hydrocarbon cracking system examples presented in FIGS. 1-4, as use of the second steam effluent exchanger 62 for second stage superheating of the high pressure steam 4 eliminates the need to provide the convection section 20 of the cracking furnace 601 with the high pressure steam superheater 25 of the previous examples.

[0067] FIG. 7 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 700 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 701 of the low-emission hydrocarbon cracking system 700. The cracking furnace 701 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 700 is also shown to include a cooling apparatus 702 including a transfer line exchanger 32 and a first and a second steam effluent exchanger 61, 62 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 701 can be cooled and by which superheated high pressure steam can be generated.

[0068] In the case of the low-emission hydrocarbon cracking system 700, the cracked effluent gas 8 may pass through each of the second steam effluent exchanger 62, the first steam effluent exchanger 61 that is downstream of the second steam effluent exchanger 62, and the transfer line exchanger 32 that is downstream of the first steam effluent exchanger 61. As in previous examples, boiler feed water 3 can be sent to a steam drum 33, and boiler water 9a may be supplied by the steam drum 33 and directed to the transfer line exchanger 32. The boiler water 9a can be at least partly vaporized in the transfer line exchanger 32 by heat recovered from the stream of cracked effluent gas 8 passing therethrough, and the partly vaporized boiler water 9b can then flow back to the steam drum 33 by natural circulation. In the steam drum 33, water can be separated from high pressure steam 4 and the high pressure steam 4 can be sent to the first steam effluent exchanger 61 where the high pressure steam 4 can be initially superheated in a first stage superheating process using heat recovered from the stream of cracked effluent gas 8 passing through the first steam effluent exchanger 61. The high pressure steam 4 initially superheated in the first stage superheating process can then be subjected to second stage superheating by directing the superheated high pressure steam 4 from the first steam effluent exchanger 61 to the second steam effluent exchanger 62 and using the waste heat in the stream of cracked effluent gas 8 passing through the second steam effluent exchanger 62 to further superheat the high pressure steam 4. A desuperheater 34 may again be used to control the temperature of the superheated high pressure steam 4 leaving the second steam effluent exchanger 62. As may be observed in FIG. 7, the flow of the high pressure steam 4 through the first steam effluent exchanger 61 and the flow of the superheated high pressure steam 4 through the second steam effluent exchanger 62 is counter-current to the flow of cracked effluent gas 8 in this example.

[0069] As may be understood from FIG. 7 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 701 of the low-emission hydrocarbon cracking system 700 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 701. Thus, the non-process duty load of the convection section 20 of the cracking furnace 701 is again minimized, and is also further reduced relative to the previous low-emission hydrocarbon cracking system examples presented in FIGS. 1-4, as use of the second steam effluent exchanger 62 for second stage superheating of the high pressure steam 4 eliminates the need to provide the convection section 20 of the cracking furnace 701 with the high pressure steam superheater 25 of the previous examples.

[0070] FIG. 8 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 800 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 801 the low-emission hydrocarbon cracking system 800. The cracking furnace 801 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 800 is also shown to include a cooling apparatus 802 including a transfer line exchanger 32 and a first and a second steam effluent exchanger 61, 62 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 801 can be cooled and by which superheated high pressure steam can be generated.

[0071] Operation of the low-emission hydrocarbon cracking system 800 is substantially the same as described relative to the low-emission hydrocarbon cracking system 700, except that the flow of the high pressure steam 4 through the first steam effluent exchanger 61 is co-current with the flow of cracked effluent gas 8 therethrough, while the flow of the superheated high pressure steam 4 through the second steam effluent exchanger 62 remains counter-current as in the low-emission hydrocarbon cracking system 700. A desuperheater 34 may again be used to control the temperature of the superheated high pressure steam 4 leaving the second steam effluent exchanger 62.

[0072] As may be understood from FIG. 8 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 801 of the low-emission hydrocarbon cracking system 800 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 801. Thus, the non-process duty load of the convection section 20 of the cracking furnace 801 is again minimized, and is also further reduced relative to the previous low-emission hydrocarbon cracking system examples presented in FIGS. 1-4, as use of the second steam effluent exchanger 62 for second stage superheating of the high pressure steam 4 eliminates the need to provide the convection section 20 of the cracking furnace 801 with the high pressure steam superheater 25 of the previous examples.

[0073] FIG. 9 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 900 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 901 the low-emission hydrocarbon cracking system 900. The cracking furnace 901 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 900 is also shown to include a cooling apparatus 902 including a transfer line exchanger 32 and a first and a second steam effluent exchanger 61, 62 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 901 can be cooled and by which superheated high pressure steam can be generated.

[0074] Operation of the low-emission hydrocarbon cracking system 900 is substantially the same as described relative to the low-emission hydrocarbon cracking system 700, except that the flow of the high pressure steam 4 through the first steam effluent exchanger 61 remains counter-current to the flow of cracked effluent gas 8 therethrough as in the low-emission hydrocarbon cracking system 700, while the flow of the superheated high pressure steam 4 through the second steam effluent exchanger 62 is co-current with the flow of cracked effluent gas 8 therethrough. A desuperheater 34 may again be used to control the temperature of the superheated high pressure steam 4 leaving the second steam effluent exchanger 62.

[0075] As may be understood from FIG. 9 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 901 of the low-emission hydrocarbon cracking system 900 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 901. Thus, the non-process duty load of the convection section 20 of the cracking furnace 901 is again minimized, and is also further reduced relative to the previous low-emission hydrocarbon cracking system examples presented in FIGS. 1-4, as use of the second steam effluent exchanger 62 for second stage superheating of the high pressure steam 4 eliminates the need to provide the convection section 20 of the cracking furnace 901 with the high pressure steam superheater 25 of the previous examples.

[0076] FIG. 10 is a schematic representation of a portion of another example configuration of a low-emission hydrocarbon cracking system in which high pressure steam is superheated at least in part by a mechanism other than a convection section of a cracking furnace of the low-emission hydrocarbon cracking system. Operation of the low-emission hydrocarbon cracking system represented in FIG. 10 is substantially the same as described relative to the low-emission hydrocarbon cracking system 100 of FIG. 1, except that the flow of the high pressure steam 4 exiting the steam effluent exchanger 60 of a cooling apparatus of the low-emission hydrocarbon cracking system is not subjected to second stage superheating in the convection section 20 of the cracking furnace, nor elsewhere. The flow of high pressure steam 4 through the steam effluent exchanger 60 is counter-current to the flow of cracked effluent gas 8 through the steam effluent exchanger 60 in this example.

[0077] FIG. 11 is a schematic representation of a portion of another example configuration of a low-emission hydrocarbon cracking system in which high pressure steam is superheated at least in part by a mechanism other than a convection section of a cracking furnace of the low-emission hydrocarbon cracking system. Operation of the low-emission hydrocarbon cracking system represented in FIG. 11 is substantially the same as described relative to the low-emission hydrocarbon cracking system 200 of FIG. 2, except that the flow of the high pressure steam 4 exiting the steam effluent exchanger 60 of a cooling apparatus of the low-emission hydrocarbon cracking system is not subjected to second stage superheating in the convection section 20 of the cracking furnace, nor elsewhere. The flow of high pressure steam 4 through the steam effluent exchanger 60 is co-current to the flow of cracked effluent gas 8 through the steam effluent exchanger 60 in this example.

[0078] FIG. 12 is a schematic representation of a portion of another example configuration of a low-emission hydrocarbon cracking system in which high pressure steam is superheated at least in part by a mechanism other than a convection section of a furnace of the low-emission hydrocarbon cracking system. Operation of the low-emission hydrocarbon cracking system represented in FIG. 12 is substantially the same as described relative to the low-emission hydrocarbon cracking system 300 of FIG. 3, except that the flow of the high pressure steam 4 exiting a steam effluent exchanger 60 of a cooling apparatus of the low-emission hydrocarbon cracking system is not subjected to second stage superheating in the convection section 20 of the cracking furnace, nor elsewhere. The flow of high pressure steam 4 through the steam effluent exchanger 60 is counter-current to the flow of cracked effluent gas 8 through the steam effluent exchanger 60 in this example.

[0079] It should be noted that in the case of each of the low-emission hydrocarbon cracking system examples presented in FIGS. 1-12, the process duty of the convection section 20 of the associated cracking furnace is unaffected by the various configurations via which the superheating of the high pressure steam 4 can be performed partially or entirely outside of the convection section 20 of the cracking furnace. Consequently, further efficiency increases to the radiant sections of the cracking furnaces of the example low-emission hydrocarbon cracking systems can be made while still allowing the low-emission hydrocarbon cracking systems to process hydrocarbon feedstocks having boiling curves with a high end point, such as condensates, gas oils, vacuum gas oils (VGO), hydrotreated vacuum gas oils (HVGO) and plastic-based oils, without a fear of fouling on the shell side of the exchanger, which is contrary to the feed effluent exchanger design of known cracking systems.

[0080] Further, the metal surface temperature of the steam effluent exchangers employed in example configurations of the low-emission hydrocarbon cracking systems according to the disclosure can be substantially higher than that of a transfer line exchanger associated with a traditional cracking furnace. As such, condensation and subsequential fouling of the heavy ends of the components in the cracked effluent gas where the dew point of the components exceeds the surface temperature of a steam effluent exchanger can be either reduced or prevented. This can be emphasized by providing a co-current high pressure steam flow path through the steam effluent exchanger, which may raise the surface temperature of the steam effluent exchanger above the dew point of the cracked effluent gas, and may be an especially effective technique at a location where the cracked effluent gas temperature is the coldest.

[0081] It should be understood that the example low-emission hydrocarbon cracking system configurations 500, 600, 700, 800, 900 illustrated in FIGS. 5-9 minimize the duty of the convection sections 20 of the cracking furnaces thereof to a greater extent than the example low-emission hydrocarbon cracking system configurations 100, 200, 300, 400 illustrated in FIGS. 1-4 because the example low-emission hydrocarbon cracking systems 500, 600, 700, 800, 900 are able to adequately superheat the high pressure steam entirely outside of the convection sections 20 of their respective cracking furnaces. Therefore, the example low-emission hydrocarbon cracking system configurations 500, 600, 700, 800, 900 also have the highest potential for reducing cracking furnace fuel consumption and associated CO 2 emissions by permitting greater efficiency improvements to the radiant section 10 of the cracking furnace. It has been determined, for example, that use of an example low-emission hydrocarbon cracking system 500, 600, 700, 800, 900 can reduce fuel consumption and CO 2 emissions by approximately 25% in comparison with a cracking furnace wherein high pressure steam superheating is performed in the convection section thereof.

[0082] In some examples, the arrangement of a steam effluent exchanger relative to a transfer line exchanger in the cracked effluent gas flow path can facilitate the use of a smaller steam effluent exchanger. For example, locating a steam effluent exchanger upstream of a transfer line exchanger instead of downstream of a transfer line exchanger may allow for a reduction in the required surface area of the steam effluent exchanger. This may be the case because when the steam effluent exchanger is upstream of the transfer line exchanger, the steam effluent exchanger receives the cracked effluent gas at a hotter temperature than would be the case in an opposite arrangement, and a lesser steam effluent exchanger surface area is resultantly required to adequately superheat the high pressure steam. The same can hold true relative to an arrangement of multiple steam effluent exchangers.

[0083] In some examples, the selection of a co-current or counter-current flow of the high pressure steam through a steam effluent exchanger can permit different steam effluent exchanger constructions. For example, the inlet area of a steam effluent exchanger is typically the hottest part of the steam effluent exchanger because the temperature of cracked effluent gas is hotter at the inlet than at the outlet or elsewhere within the steam effluent exchanger. The high temperature of the cracked effluent gas can push the functional limits of the material from which the steam effluent exchanger is constructed. At least in such a case, employing a co-current steam effluent exchanger design can reduce the maximum steam effluent exchanger tube wall temperature and, in turn, the required tube wall thickness. Consequently, steam effluent exchanger materials may be maintained within a safe operating range, or construction materials that may not otherwise be usable may be usable, by employing a co-current steam effluent exchanger design.

[0084] FIG. 13 is a flowchart 1000 describing one example of a method of operating a low-emission hydrocarbon cracking system to convert a hydrocarbon feedstock into a cracked effluent gas. As indicated in block 1002 a hydrocarbon feedstock can be preheated in a convection section of a cracking furnace of the low-emission hydrocarbon cracking system. The convection section may be positioned to receive a flow of flue gas generated by a combustion of fuel in a radiant section of the cracking furnace. The convection section may include a number of convection banks that can be utilized for various purposes, such as to perform other process or non-process (convection) heating operations. The heating operations for which the convection section is responsible defines the required duty capacity of the convection section.

[0085] As indicated in block 1004 the cracked effluent gas can be generated by heating the preheated hydrocarbon feedstock in at least one radiant coil arranged in the radiant section of the cracking furnace. In some examples, the cracked effluent gas may be produced by heating the preheated hydrocarbon feedstock in a radiant section of the cracking furnace such that the preheated hydrocarbon feedstock is exposed to sufficient combustion heat to cause a pyrolysis of the hydrocarbon feedstock. In other examples, the heat required to crack the hydrocarbon feedstock may be produced in the radiant section by electric heating elements instead of by fuel combustion.

[0086] As indicated in block 1006 the cracked effluent gas may be cooled upon exiting the radiant section of the cracking furnace. Cooling of the cracked effluent gas leaving the radiant section may be performed, for example, to stop pyrolysis side reactions, and to preserve the equilibrium of the reactions in favor of the products generated during pyrolysis. In some examples, cooling of the cracked effluent gas can be accomplished, at least in part, using a transfer line exchanger.

[0087] As indicated in block 1008, at least partly vaporized boiler water can be generated by heating boiler water in the transfer line exchanger using waste heat from the cracked effluent gas. Vaporizing the boiler water using waste heat from the cracked effluent gas simultaneously cools the cracked effluent gas. The boiler water may be supplied from, for example, a steam drum that is in fluid communication with the transfer line exchanger. When present, the steam drum may also receive a flow of boiler feed water to replenish the boiler water supply. As indicated in block 1010, high pressure steam can be generated, at least in part, from the at least partly vaporized boiler water generated by the transfer line exchanger.

[0088] As indicated in block 1012, superheated high pressure steam can be generated by heating a flow of the high pressure steam in a steam effluent exchanger using waste heat from the cracked effluent gas. The steam effluent exchanger may be placed in a flow path of the cracked effluent gas, such that the hot cracked effluent gas passes through the steam effluent exchanger. The steam effluent exchanger may be arranged in series with the transfer line exchanger. The steam effluent exchanger may be located upstream or downstream from the transfer line exchanger. When multiple steam effluent exchangers are used, one or more steam effluent exchangers may be located upstream of the transfer line exchanger and one or more steam effluent exchangers may be located downstream of the transfer line exchanger. In another example, all of the multiple steam effluent exchangers can be located upstream of the transfer line exchanger. The flow of the high pressure steam through a steam effluent exchanger may be co-current with or counter-current to the flow of the cracked effluent gas through the steam effluent exchanger. When multiple steam effluent exchangers are used, the flow of the high pressure steam through each of the steam effluent exchangers may be co-current with, counter-current to, or some combination of co-current with and counter-current to, the flow of the cracked effluent gas through the steam effluent exchangers.

[0089] While the various examples of low-emission hydrocarbon cracking systems presented herein all include a radiant section that relies on the combustion of a fuel to generate the heat necessary to crack the hydrocarbon feedstock, it should be understood that the cracking heat may be generated electrically in other examples. That is, the low-emission cracking furnace examples may be electric cracking furnaces. In still other examples, cracking of the hydrocarbon feedstock may be performed using a rotodynamic device, which is a device capable of causing the pyrolysis reaction of the hydrocarbon feedstock using kinetic energy generated via static and dynamic rotors. The use of other radiant section designs may also be possible.

[0090] The foregoing description of certain examples, including illustrated examples, has been presented only for the purpose of illustration and description and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Numerous modifications, adaptations, and uses thereof will be apparent to those skilled in the art without departing from the scope of the disclosure.

Examples

Embodiment Construction

[0024]Certain aspects and examples of the present disclosure relate to a low-emission hydrocarbon cracking system that includes a cracking furnace having a convection section and a radiant section and is operable to crack a hydrocarbon feedstock. The low-emission hydrocarbon cracking system may achieve lower emissions by, for example, facilitating the ability to increase the efficiency of the radiant section (e.g., firebox) of the cracking furnace without exceeding a duty capacity of a convection section of the cracking furnace. This may be achieved in some examples by utilizing configurations where the process of superheating high pressure steam used for power generation or other downstream processes is performed partly or completely outside the convection section of the cracking furnace. Superheating the high pressure steam outside the convection section lessens the heat demand of the convection section on the flue gas that is generated during the hydrocarbon cracking process. Thi...

Technical Field

[0001] The present disclosure relates generally to a low-emission hydrocarbon cracking system, and more particularly although not necessarily exclusively, to a low-emission hydrocarbon cracking system in which superheating of high pressure steam can be accomplished at least partially outside of a convection section of a furnace of the hydrocarbon cracking system.Background

[0002] A cracking furnace may be used to crack (break down the molecules of) hydrocarbons. The cracking process can allow heavier hydrocarbons to be broken down into more useful lighter hydrocarbons. One type of cracking furnace is a steam cracking furnace, wherein a hydrocarbon feedstock may be introduced into the furnace, diluted with steam, and heated in a radiant section such as a firebox to a sufficiently high temperature to crack the hydrocarbon feedstock. The hydrocarbon feedstock can be converted into various useful products, including among others, ethylene and propylene.Summary

[0003] Example No. 1 is a hydrocarbon cracking system for converting a hydrocarbon feedstock into a cracked effluent gas. The hydrocarbon cracking system includes a furnace, which comprises a convection section having at least one hydrocarbon feedstock preheater, a radiant section comprising at least one radiant coil arranged to receive the preheated hydrocarbon feedstock and to generate the cracked effluent gas therefrom, and a cooling apparatus arranged to receive the cracked effluent gas. The cooling apparatus comprises a transfer line exchanger arranged to receive a boiler water flow and to generate at least partly vaporized boiler water therefrom using waste heat from the cracked effluent gas, and a steam effluent exchanger arranged to receive a high pressure steam flow and to generate superheated high pressure steam therefrom using waste heat from the cracked effluent gas.

[0004] Example No. 2 is Example No. 1, further comprising a high pressure steam source that receives the at least partly vaporized boiler water from the transfer line exchanger. The high pressure steam source is in fluid communication with the steam effluent exchanger.

[0005] Example No. 3 is Example No. 2, wherein the steam effluent exchanger is arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas. The steam effluent exchanger is located either downstream or upstream of the transfer line exchanger, and a flow path of the high pressure steam flow through the steam effluent exchanger is either counter-current to or co-current with the flow path of the cracked effluent gas.

[0006] Example No. 4 is Example No. 2, further comprising a high pressure steam superheater located in the convection section of the furnace and arranged to receive the superheated high pressure steam from the steam effluent exchanger and to further superheat the superheated high pressure steam.

[0007] Example No. 5 is Example No. 2, wherein the steam effluent exchanger comprises at least two steam effluent exchangers. A first steam effluent exchanger of the at least two steam effluent exchangers is arranged to receive the high pressure steam flow from the high pressure steam source and to generate the superheated high pressure steam therefrom. A second steam effluent exchanger of the at least two steam effluent exchangers is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam. A desuperheater is located in a flow path of the superheated high pressure steam between the first steam effluent exchanger and the second steam effluent exchanger and is operable to control a temperature of the superheated high pressure steam leaving the second steam effluent exchanger by injecting cold boiler feed water into the superheated high pressure steam.

[0008] Example No. 6 is Example No. 2, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger is located downstream of the transfer line exchanger and the second steam effluent exchanger is located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas. The second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

[0009] Example No. 7 is Example No. 2, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger is located downstream of the transfer line exchanger and the second steam effluent exchanger is located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is co-current with the flow path of the cracked effluent gas The second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

[0010] Example No. 8 is Example No. 2, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas. The first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas. The second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

[0011] Example No. 9 is Example No. 2, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas. The first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is co-current with the flow path of the cracked effluent gas. The second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

[0012] Example No. 1 0 is Example No. 2, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with a transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas. The first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas. The second steam effluent exchanger is configured to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is co-current with the flow path of the cracked effluent gas.

[0013] Example No. 11 is a heat exchanger and cracking furnace arrangement. The heat exchanger and cracking furnace arrangement includes a transfer line exchanger positioned in a cracked effluent gas flow path from a cracking furnace, and a steam effluent exchanger positioned in the cracked effluent gas flow path in series with the transfer line exchanger. The transfer line exchanger comprises a boiler water input and an at least partly vaporized boiler water output, and is configured to at least partly vaporize the boiler water using waste heat from the cracked effluent gas. The steam effluent exchanger comprises a high pressure steam input and a superheated high pressure steam output, and is configured to generate the superheated high pressure steam using waste heat from the cracked effluent gas.

[0014] Example No. 12 is Example No. 11, wherein the steam effluent exchanger comprises at least two steam effluent exchangers. A first steam effluent exchanger of the at least two steam effluent exchangers is arranged to receive a high pressure steam flow from a high pressure steam source and to generate the superheated high pressure steam therefrom. A second steam effluent exchanger of the at least two steam effluent exchangers is positioned to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and is configured to further superheat the superheated high pressure steam. The second steam effluent exchanger is configured to direct the flow of the superheated high pressure steam through the second steam effluent exchanger along a path that is co-current with or counter-current to a path of the high pressure steam flow through the first steam effluent exchanger.

[0015] Example No. 13 is Example No. 12, further comprising a desuperheater that is positioned in a flow path of the superheated high pressure steam between the first steam effluent exchanger and the second steam effluent exchanger and is operable to control a temperature of the superheated high pressure steam leaving the second steam effluent exchanger by injecting cold boiler feed water into the superheated high pressure steam.

[0016] Example No. 14 is a method of operating a hydrocarbon cracking system to convert a hydrocarbon feedstock into a cracked effluent gas. The method includes preheating the hydrocarbon feedstock in a convection section of a cracking furnace of the hydrocarbon cracking system, and generating a cracked effluent gas by heating the preheated hydrocarbon feedstock in a radiant section of the cracking furnace. The method also includes cooling the cracked effluent gas by passing the cracked effluent gas through a transfer line exchanger and generating at least partly vaporized boiler water by heating boiler water in the transfer line exchanger using waste heat from the cracked effluent gas. The method further includes generating high pressure steam, at least in part, from the at least partly vaporized boiler water, and generating superheated high pressure steam by heating a flow of the high pressure steam in a steam effluent exchanger using waste heat from the cracked effluent gas.

[0017] Example No. 15 is Example No. 14, wherein the high pressure steam is provided by a steam drum from a combination of the boiler water present in the steam drum and the at least partly vaporized boiler water generated in the transfer line exchanger, and the flow of the high pressure steam is received by the steam effluent exchanger from the steam drum.

[0018] Example No. 16 is Example No. 14, wherein the steam effluent exchanger is arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas, the steam effluent exchanger is located either downstream or upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the high pressure steam flows through the steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas.

[0019] Example No. 17 is Example No. 14, wherein the superheated high pressure steam generated in the steam effluent exchanger is further superheated by directing the superheated high pressure steam to a high pressure steam superheater located in the convection section of the cracking furnace.

[0020] Example No. 18 is Example No. 14, wherein the steam effluent exchanger comprises at least two steam effluent exchangers. A first steam effluent exchanger of the at least two steam effluent exchangers receives the flow of the high pressure steam from a high pressure steam source and generates the superheated high pressure steam therefrom. A second steam effluent exchanger of the at least two steam effluent exchangers receives a flow of the superheated high pressure steam from the first steam effluent exchanger and further superheats the superheated high pressure steam. A desuperheater is also located in a flow path of the superheated high pressure steam between the first steam effluent exchanger and the second steam effluent exchanger and is used to control a temperature of the superheated high pressure steam leaving the second steam effluent exchanger by injecting cold boiler feed water into the superheated high pressure steam.

[0021] Example No. 19 is Example No. 14, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger, and the first steam effluent exchanger and the second steam effluent exchanger are arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger is located downstream of the transfer line exchanger along the flow path of the cracked effluent gas and the second steam effluent exchanger is located upstream of the transfer line exchanger along the flow path of the cracked effluent gas. The superheated high pressure steam is generated in the first steam effluent exchanger by directing the flow of the high pressure steam through the first steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas. The superheated high pressure steam generated in the first steam effluent exchanger is further superheated in the second steam effluent exchanger by directing a flow of the superheated high pressure steam through the second steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas.

[0022] Example No. 20 is Example No. 14, wherein the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger, and the first steam effluent exchanger and the second steam effluent exchanger are arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas. The first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas. The superheated high pressure steam is generated in the first steam effluent exchanger by directing the flow of the high pressure steam through the first steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas. The superheated high pressure steam generated in the first steam effluent exchanger is further superheated in the second steam effluent exchanger by directing a flow of the superheated high pressure steam through the second steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas.Brief Description of the Drawings

[0023] FIG. 1 is a schematic representation of a low-emission hydrocarbon cracking system according to one example configuration. FIG. 2 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 3 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 4 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 5 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 6 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 7 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 8 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 9 is a schematic representation of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 10 is a schematic representation of a portion of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 11 is a schematic representation of a portion of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 12 is a schematic representation of a portion of another low-emission hydrocarbon cracking system according to another example configuration. FIG. 13 is a flowchart describing one example of a method of cracking a hydrocarbon feedstock using a low-emission hydrocarbon cracking system. Detailed Description

[0024] Certain aspects and examples of the present disclosure relate to a low-emission hydrocarbon cracking system that includes a cracking furnace having a convection section and a radiant section and is operable to crack a hydrocarbon feedstock. The low-emission hydrocarbon cracking system may achieve lower emissions by, for example, facilitating the ability to increase the efficiency of the radiant section (e.g., firebox) of the cracking furnace without exceeding a duty capacity of a convection section of the cracking furnace. This may be achieved in some examples by utilizing configurations where the process of superheating high pressure steam used for power generation or other downstream processes is performed partly or completely outside the convection section of the cracking furnace. Superheating the high pressure steam outside the convection section lessens the heat demand of the convection section on the flue gas that is generated during the hydrocarbon cracking process. This reduction in convection section heat demand can ensure that the convection section still has an adequate duty capacity even if the amount of heat that is providable thereto by the flue gas is reduced as a result of increasing the efficiency of the radiant section.

[0025] A low-emission hydrocarbon cracking system according to an example configuration may generally include a convection section, in which hydrocarbon feedstock can be preheated and / or partly evaporated and mixed with dilution steam to provide a feedstock-dilution steam mixture. The system can also comprise a radiant section, which may include, in some examples, at least one radiant coil in a firebox, and in which the feedstock-dilution steam mixture from the convection section can be converted into product and byproduct components at high temperature by pyrolysis. A hydrocarbon cracking system example can further comprise a cooling apparatus including at least one transfer line exchanger, that may be configured to quickly quench the product or cracked effluent gas leaving the radiant section of the cracking furnace in order to stop pyrolysis side reactions, and to preserve the equilibrium of the reactions in favor of the products. Heat from the transfer line exchanger can be recovered in the form of the high pressure steam.

[0026] In an example configuration of a low-emission hydrocarbon cracking system, the convection section may include multiple convection banks for preheating the feedstock and for other purposes, such as for example, heating boiler water, heating a mixture of the feedstock and dilution steam, superheating the dilution steam, and superheating the high pressure steam. The convection section banks receive heat from flue gas that is generated in the radiant section and flows through the convection section. Each of the operations performed by the convection banks of the convection section are part of the duty of the convection section and the convection section needs to have sufficient duty capacity to perform the operations.

[0027] In some examples, the required non-process duty of the convection section can be minimized by performing in another manner, one or more operations typically performed in the convection section. For example, the non-process duty of the convection section can be minimized by superheating high pressure steam outside the convection section using non-convection section components. Removing part or all of the high pressure steam superheating responsibility from the convection section allows the convection section duty to be minimized without affecting the process duty, which includes the duty related to preheating or preheating and vaporizing the feedstock, heating the feedstock-dilution steam mixture, and superheating the dilution steam. With the convection section non-process duty minimized, the radiant section (e.g., fire box) efficiency can be increased without concern for high pressure steam superheating limitations of the convection section that could otherwise negatively impact the quality of high pressure steam that is available for effective downstream power generation. For example, and as described in more detail below, insufficiently superheated high pressure steam can cause condensation problems in downstream turbines to which the superheated high pressure steam is directed.

[0028] Example configurations of a low-emission hydrocarbon cracking system may use the heat of the effluent produced by the pyrolysis process (i.e., the cracked effluent gas) to superheat the high pressure steam instead of using heat from the flue gas. For example, one or more steam effluent exchangers may be placed in a flow path of the cracked effluent gas that is discharged from the radiant section of the cracking furnace. High pressure steam, such as from a steam drum or another high pressure steam source, may be directed through the one or more steam effluent exchangers to superheat the high pressure steam using the waste heat of the cracked effluent gas that also flows therethrough.

[0029] In some examples, only one steam effluent exchanger may be used, and the steam effluent exchanger may be located upstream or downstream of the transfer line exchanger used to cool the cracked effluent gas stream.

[0030] In some examples, the superheated high pressure steam may be discharged from the steam effluent exchanger for downstream use. In other examples, superheating of the high pressure steam may be a two-stage process, where the steam effluent exchanger is used in a first stage high pressure steam superheating operation. In such an example, a second stage high pressure steam superheating operation may be performed by directing the initially superheated high pressure steam from the steam effluent exchanger to a high pressure steam superheater in the convection section.

[0031] In some examples, more than one steam effluent exchanger may be used to superheat the high pressure steam. In such an example, the steam effluent exchangers may be placed in different arrangements relative to the transfer line exchanger. For example, when two steam effluent exchangers are used, one steam effluent exchanger may be located upstream of the transfer line exchanger and the other steam effluent exchanger may be located downstream of the transfer line exchanger. In another example configuration in which two steam effluent exchangers are used, both of the steam effluent exchangers may be located upstream of the transfer line exchanger.

[0032] In some examples, the flow of high pressure steam through a steam effluent exchanger may be co-current with the flow of the cracked effluent gas through the steam effluent exchanger. In other examples, the flow of high pressure steam through a steam effluent exchanger may be counter-current to the flow of the cracked effluent gas through the steam effluent exchanger. In an example where more than one steam effluent exchanger is used to superheat high pressure steam, the flow of high pressure steam through each steam effluent exchanger may be co-current with the flow of the cracked effluent gas therethrough, counter-current to the flow of the cracked effluent gas therethrough, or may be co-current with respect to one or some of the steam effluent exchangers and counter-current to one or some of the other steam effluent exchangers.

[0033] When more than one steam effluent exchanger is used, the arrangement of the steam effluent exchangers relative to each other can reduce the required surface area of one or some of the steam effluent exchangers. The arrangement of the steam effluent exchangers relative to each other may also allow for the use of previously unusable steam effluent exchanger materials by reducing the maximum tube wall temperature that will be experienced by a steam effluent exchanger.

[0034] In some examples, the pressure of the high pressure steam generated by a low-emission hydrocarbon cracking system may be, for example, between 15 bar and 130 bar. In some examples, the temperature of the superheated high pressure steam generated by the one or more steam effluent exchangers may vary depending on the intended downstream use of the steam. For example, if the downstream steam system contains one or more condensing turbines, the temperature of the superheated high pressure steam can be controlled so that the resulting superheated high pressure steam does not produce condensation in the condensing turbines that exceeds a maximum allowable condensation percentage. In another example, where the downstream steam system includes only a back pressure turbine(s), the temperature of the superheated high pressure steam can be controlled so that the resulting superheated high pressure steam does not produce condensation in the back pressure turbine(s). Generally speaking, the high pressure steam will typically require less superheating when it will be directed to a back pressure turbine(s) than when it will be directed to a condensing turbine(s). In another example, where the high pressure steam is used as heating medium, there may be no such restrictions, but the temperature of the superheated high pressure steam should nonetheless be sufficient to prevent steam condensation in steam supply lines located upstream of heat exchangers to which the high pressure steam is directed. In some examples, the temperature of the superheated high pressure steam can be between 200 °C to 550 °C, depending on the pressure of the high pressure steam and the amount of superheating required to prevent turbine condensation damage. In one example, the temperature of the superheated high pressure steam may be about 400 °C.

[0035] Illustrative examples follow and are given to introduce the reader to the general subject matter discussed herein rather than to limit the scope of the disclosed concepts. The following sections describe various additional features and examples with reference to the drawings in which like numerals indicate like elements, and directional descriptions are used to describe the illustrative aspects, but, like the illustrative aspects, should not be used to limit the present disclosure.

[0036] FIG. 1 is a schematic representation of one example configuration of a low-emission hydrocarbon cracking system 100 in which high pressure steam 4 is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 101 of the low-emission hydrocarbon cracking system 100. As shown, during operation of the low-emission hydrocarbon cracking system 100, a hydrocarbon feedstock 1 can be introduced to a feed preheater 22, which may be one convection bank 21 of multiple convection banks 21 in a convection section 20 of the cracking furnace 101. The hydrocarbon feedstock 1 can be one of various usable hydrocarbons, and may be paraffinic or naphthenic in nature. Small quantities of aromatics and olefins can also be present. For example, the hydrocarbon feedstock may be ethane, propane, butane, natural gasoline, naphtha, kerosene, natural gas condensate, gas oil, vacuum gas oil, hydrotreated or desulphurized or hydrodesulphurized (vacuum) gas oils, or combinations thereof.

[0037] The convection section 20 of the cracking furnace 101 may more specifically include, from top to bottom in this example, a stack 31 that employs a flue gas fan 30 to evacuate flue gas 7, and multiple convection banks 21 that include: an air preheater 27 that is usable to preheat combustion air; the feed preheater 22; a first high temperature coil 23 that is usable to heat a mixture of the hydrocarbon feedstock 1 and dilution steam 2; a dilution steam superheater 24 that is usable to superheat the dilution steam 2; a high pressure steam superheater 25 that is usable to superheat saturated high pressure steam; and a second high temperature coil 26 that is usable to heat the mixture of the hydrocarbon feedstock 1 and dilution steam 2. The type, number, and arrangement of the convection banks 21 may be different in other cracking furnace examples.

[0038] The hydrocarbon feedstock 1 may be preheated or partially or fully evaporated in the feed preheater 22 before being mixed with the dilution steam 2. The dilution steam 2 can be injected directly, or may be superheated before being mixed with the hydrocarbon feedstock 1. In this example, the dilution steam 2 can be superheated in the dilution steam superheater 24 prior to being mixed with the hydrocarbon feedstock 1. In some cases, multiple steam injection points may be provided to help dilute heavier hydrocarbon feedstocks.

[0039] A mixture of the hydrocarbon feedstock 1 and the dilution steam 2 can be further heated by the high temperature coils 23, 26 to reach an optimum temperature prior to introduction to a radiant section (i.e., firebox) 10. Within the firebox 10 is a radiant coil 11 within which the hydrocarbon feedstock 1 travels and is quickly heated to a temperature at which a pyrolysis reaction occurs. The heat of reaction for the pyrolysis reaction, which is highly endothermic, can be supplied by the combustion of fuel (e.g., gas) 5 with combustion air 6 in the firebox 10. Combustion can be performed by bottom burners 12, side wall burners (not shown), or a combination thereof. The radiant coil 11 may run a length of the firebox 10 to distribute the heat as evenly as possible to the radiant coil and the hydrocarbon feedstock located therein. The efficiency of this example of the cracking furnace 101 is determined by the heat release pattern of the bottom burners 12 and the extent of the air preheating.

[0040] Combustion air 6 can be provided by a forced draft combustion air fan 37, which can force the combustion air 6 through an air preheater 27 situated in the convection section 20 where the combustion air 6 can recover waste heat from the flue gas 7. The preheated combustion air 6 can then be sent to the bottom burners 12 of the firebox 10 to combust the fuel 5. This produces a hot combustion zone 14. The efficiency of the firebox 10 can be improved by locating the combustion zone 14 lower in the firebox 10 and increasing the combustion zone temperature. The bottom burners 12 provide heat low in the firebox 10 and preheating of the combustion air 6 increases the temperature of the combustion zone 14. Further, liberating more flue gas lower in the firebox 10 makes the flue gas flow more co-current compared to liberating the flue gas more evenly throughout the firebox 10, and also results in a greater interaction between the hot flue gas and the radiant coil 11. The hotter combustion temperature raises the temperature driving force, especially improving the radiative heat transfer. Both the restriction of the combustion zone 14 to the lower part of the firebox 10 and the preheating of the combustion air can be used to improve firebox efficiency, thus reducing fuel consumption and associated CO 2 emissions. In another example (not shown), a similar result may be obtained by locating multiple rows of side wall burners with air preheat in a lower portion of the vertical walls of the firebox 10 in lieu of the bottom burners 12.

[0041] Through the pyrolysis reaction, the hydrocarbon feedstock is converted into products and by-products. Examples of such products can include hydrogen, ethylene, propylene, butadiene, benzene, toluene, styrene and xylenes. Byproducts may include methane and fuel oil. The resulting mixture of the dilution steam 2, unconverted hydrocarbon feedstock 1, and converted feedstock, creates a reactor effluent that may be referred to as "cracked effluent gas" 8. The cracked effluent gas 8 can be cooled in a cooling apparatus 102 of the low-emission hydrocarbon cracking system 100 in an effort to maintain the equilibrium of the pyrolysis reactions in favor of the products rather than the byproducts.

[0042] At least some of the waste heat in the cracked effluent gas can be recovered in a transfer line exchanger 32 of the cooling apparatus 102 of the low-emission hydrocarbon cracking system 100 and may be used to generate saturated high pressure steam 4. The high pressure steam 4 can be generated, for example, using boiler water 9a from a steam drum 33. Boiler feed water 3 may be provided to the steam drum 33 for this purpose. The boiler feed water 3 can mix with boiler water 9 already present in the steam drum 33. The boiler water 9a from a steam drum 33 is partly vaporized in the transfer line exchanger 32. The partly vaporized boiler water 9b can flow back to the steam drum 33 by way of natural circulation. Additionally, the high pressure steam 4 can be separated from the boiler water in the steam drum 33. To control the high pressure steam temperature, boiler feed water may be injected in a desuperheater 34.

[0043] Combustion of the fuel 5 with the combustion air 6 in the radiant section 10 of the cracking furnace 101, generates combustion products such as water and CO 2 , which is commonly referred to as "flue gas" 7. Waste heat from the flue gas 7 may be recovered in the convection section 20 using various types of the convection banks 21. Some of the waste heat can be used for process side purposes, such as for preheating and / or evaporating the hydrocarbon feedstock 1 and the dilution steam 2. The remainder of the waste heat may be used for the non-process side purposes, such as to generate the high pressure steam 4.

[0044] It is possible in some cases to superheat the high pressure steam using one or more convection banks in the convection section 20 of the furnace 101 using the flue gas 7 as the heating medium. For example, when sufficient heat is generated in the firebox 10, enough of the heat may be recoverable in the convection section 20 to provide the convection section 20 with the available convection duty required to perform process convection duties and to also superheat the generated high pressure steam 4. However, if the firebox efficiency is increased, which is a desirable improvement, at least the non-process duty of the convection section 20 will be significantly reduced and there may not be sufficient heat in the flue gas 7 to allow the convection section 20 to fulfill its process duty functions and also superheat the high pressure steam 4.

[0045] The low-emission hydrocarbon cracking system 100, as well as other low-emission hydrocarbon system examples according to the disclosure, can thus be configured to allow for additional improvements in radiant section efficiency while still ensuring sufficient process and non-process duty capacity in the convection section 20. In the low-emission hydrocarbon cracking system 100 configuration of FIG. 1 (and in the other low-emission hydrocarbon cracking system examples that follow), a non-process duty of the convection section 20 of the cracking furnace 101 can be minimized by removing at least a portion of the high pressure steam superheating burden therefrom. To this end, the cooling apparatus 102 of the low-emission hydrocarbon cracking system 100 can further include a at least one steam effluent exchanger 60 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 101 can be further cooled, and by which superheated high pressure steam can be generated using waste heat from the cracked effluent gas 8.

[0046] In the case of the low-emission hydrocarbon cracking system 100, the cracked effluent gas 8 generated in the radiant section 10 of the cracking furnace 101 can be initially cooled in the transfer line exchanger 32 and then further cooled in the steam effluent exchanger 60 that is downstream of the transfer line exchanger 32 relative to the flow path of the cracked effluent gas 8 upon its exit from the radiant section 10. Boiler feed water 3 can be sent to a steam drum 33, and boiler water 9a from the steam drum 33 may be directed to the transfer line exchanger 32. The boiler water 9a can be at least partly vaporized in the transfer line exchanger 32 by heat recovered from the stream of cracked effluent gas 8 passing through the transfer line exchanger 32. The partly vaporized boiler water 9b can then flow back to the steam drum 33 by natural circulation. In the steam drum 33, water can be separated from high pressure steam 4 and the high pressure steam 4 can be sent to the steam effluent exchanger 60 where the high pressure steam 4 can be superheated by heat recovered from the stream of cracked effluent gas 8 passing through the steam effluent exchanger 60. As shown, the flow of the high pressure steam 4 through the steam effluent exchanger 60 may be counter-current to the flow of cracked effluent gas 8 in this example.

[0047] As may be understood from FIG. 1 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 101 of the low-emission hydrocarbon cracking system 100 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 101. This helps to minimize the non-process duty load of the convection section 20 of the cracking furnace 101.

[0048] In the low-emission hydrocarbon cracking system 100, superheating of the high pressure steam 4 in the steam effluent exchanger 60 may be a first stage of high pressure steam superheating. In this example, a second stage of high pressure steam superheating can still be performed by directing the high pressure steam 4 discharged from the steam effluent exchanger 60 to the high pressure steam superheater 25 in the convection section 20 of the cracking furnace 101 to be further superheated using heat from the flue gas 7 that travels from the radiant section 10 through the convection section 20. To control the temperature of the high pressure steam 4 directed to the high pressure steam superheater 25 from the steam effluent exchanger 60, the high pressure steam may be passed through a desuperheater 34 prior to its introduction to the high pressure steam superheater 25. The desuperheater 34 may operate to cool the high pressure steam by injecting cold boiler feed water into the high pressure steam 4. Use of the desuperheater 34 in this manner can allow for precise control over the superheating of the high pressure steam 4 leaving the high pressure steam superheater 25. This can be beneficial to power generation, especially when the downstream steam system utilizes condensing turbines, as it generally does in ethylene plants.

[0049] A reduction in fuel consumption and a corresponding decrease in CO 2 emissions associated with the low-emission hydrocarbon cracking system 100 and the other low-emission hydrocarbon cracking system examples that follow can be facilitated as described in detail above - by improving firebox efficiency through restriction of the combustion zone 14 to the lower part of the firebox 10 and preheating of the combustion air 6. When fuel consumption is reduced (i.e., less fuel is burned), flue gas production is also reduced. The production of less flue gas 7 results in a reduced convection duty of the convection section 20. If the resulting reduced convection duty of the convection section 20 becomes insufficient to satisfy the required process and non-process duty requirements associated with operation of the cracking furnace 101, then the convection duty of the convection section needs to be reduced in order to ensure proper cracking furnace operation.

[0050] One way to reduce the convection duty of the convection section 20 is to utilize a feed effluent exchanger that uses a mixture of feedstock and dilution steam on the shell side of the exchanger instead of high pressure steam, which can reduce the convection section process duty heat requirements. However, the feedstock itself can be subject to fouling reactions and the feedstock-dilution steam mixture must enter the shell side of the exchanger in the vapor phase to prevent fouling by contact between liquid droplets and the hot tube surface. As some feedstocks can be relatively heavy and can have a boiling range with a high end point, it can be a challenge to fully evaporate these feedstocks at a temperature that is low enough to make the feed effluent exchanger an effective tool for removing sufficient convection duty from the convection section.

[0051] Alternatively, and as described in further detail below, the non-process duty heat requirements of the convection section 20 can be reduced by utilizing one or more steam effluent exchangers that can be arranged to receive a high pressure steam flow and configured to generate superheated high pressure steam from the high pressure steam flow using waste heat from the cracked effluent gas produced by the cracking furnace 101. This can remove or at least reduce the high pressure steam superheating requirements of the convection section 20, thereby allowing the cracking furnace 101 to operate properly even when flue gas generation is decreased as a result of reduced fuel consumption. Further, with saturated high pressure steam on the shell side of the steam effluent exchanger, sufficient throughput on both sides of the exchanger can be ensured and shell-side fouling can be avoided. This can also permit a low-emission hydrocarbon cracking system according to the present disclosure to utilize a broad range of hydrocarbon feedstocks without shell-side fouling concerns.

[0052] In the drawing figures associated with the following low-emission hydrocarbon cracking system configuration examples of FIGS. 2-9, at least redundant portions of the cracking furnace 101 of FIG. 1 have been reused for clarity and to better illustrate differences between the various low-emission hydrocarbon cracking system configuration examples presented herein. Components of the low-emission hydrocarbon cracking system configuration examples illustrated in FIGS. 2-9 that have commonality with the components of the cracking furnace 101 of FIG. 1 are indicated by the same reference numbers used in FIG. 1.

[0053] FIG. 2 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 200 in which high pressure steam is superheated at least in part by a mechanism other than a convection section of a cracking furnace 201 of the low-emission hydrocarbon cracking system 200. The cracking furnace 201 of the low-emission cracking furnace 200 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 200 is also shown to include a cooling apparatus 202 including a transfer line exchanger 32 and a steam effluent exchanger 60 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 201 can be cooled and by which superheated high pressure steam can be generated. Operation of the low-emission hydrocarbon cracking system 200 is substantially the same as described relative to the low-emission hydrocarbon cracking system 100, except that the flow of high pressure steam 4 through the steam effluent exchanger 60 of the cooling apparatus 202 of the low-emission hydrocarbon cracking system 200 is co-current to the flow of cracked effluent gas 8 through the steam effluent exchanger 60 in this example instead of counter-current as in the low-emission hydrocarbon cracking system 100.

[0054] As may be understood from FIG. 2 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 201 of the low-emission hydrocarbon cracking system 200 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 201. This again helps to minimize the non-process duty load of the convection section 20.

[0055] FIG. 3 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 300 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 301 of the low-emission hydrocarbon cracking system 300. The cracking furnace 301 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 300 is also shown to include a cooling apparatus 302 including a transfer line exchanger 32 and a steam effluent exchanger 60 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 301 can be cooled and by which superheated high pressure steam can be generated.

[0056] In the low-emission hydrocarbon cracking system 300, the heat of the cracked effluent gas 8 stream can initially be used to superheat high pressure steam 4 in the steam effluent exchanger 60 prior to the cracked effluent gas 8 being further cooled in the transfer line exchanger 32 that is in series with and downstream of the steam effluent exchanger 60 along the flow path of the cracked effluent gas 8. Boiler feed water 3 can be sent to a steam drum 33 and boiler water 9a from the steam drum 33 may be directed to the transfer line exchanger 32. The boiler water 9a can be at least partly vaporized in the transfer line exchanger 32 by heat recovered from the stream of cracked effluent gas 8 passing therethrough. The partly vaporized boiler water 9b can then flow back to the steam drum 33 by natural circulation. In the steam drum 33, water can be separated from high pressure steam 4 and the high pressure steam 4 can be sent to the steam effluent exchanger 60 for superheating of the high pressure steam 4 in steam effluent exchanger 60 in the manner previously described with respect to the low-emission hydrocarbon cracking system 100 of FIG. 1. As shown, the flow of the high pressure steam 4 through the steam effluent exchanger 60 may again be counter-current to the flow of cracked effluent gas 8 therethrough.

[0057] As may be understood from FIG. 3 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 301 of the low-emission hydrocarbon cracking system 300 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 301, and resultantly helps to minimize the non-process duty load of the convection section 20 of the cracking furnace 301. Likewise, superheating of the high pressure steam 4 in the steam effluent exchanger 60 may again be a first stage of high pressure steam superheating, and the superheated high pressure steam may be passed to the high pressure steam superheater 25 in the convection section 20 of the cracking furnace 301 for further (second stage) superheating as described above with respect to the low-emission hydrocarbon cracking system 100 of FIG. 1. A desuperheater 34 may be used to control the temperature of the superheated high pressure steam leaving the high pressure steam superheater 25.

[0058] FIG. 4 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 400 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 401 of the low-emission hydrocarbon cracking system 400. The cracking furnace 401 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 400 is also shown to include a cooling apparatus 402 including a transfer line exchanger 32 and a steam effluent exchanger 60 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 401 can be cooled and by which superheated high pressure steam can be generated. Operation of the low-emission hydrocarbon cracking system 400 is substantially similar to the operation of the low-emission hydrocarbon cracking system 300 of FIG. 3, as the low-emission hydrocarbon cracking system 400 also has a steam effluent exchanger 60 arranged upstream of a transfer line exchanger 32. As may be observed, the flow of the high pressure steam 4 through the steam effluent exchanger 60 is co-current to the flow of cracked effluent gas 8 therethrough in this example instead of counter-current as in the low-emission hydrocarbon cracking system 300.

[0059] As may be understood from FIG. 4 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 401 of the low-emission hydrocarbon cracking system 400 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 401. The non-process duty load of the convection section 20 of the cracking furnace 401 may thus again be minimized.

[0060] FIG. 5 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 500 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 501 the low-emission hydrocarbon cracking system 500. The cracking furnace 501 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 500 is also shown to include a cooling apparatus 502 including a transfer line exchanger 32 and a first and a second steam effluent exchanger 61, 62 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 501 can be cooled and by which superheated high pressure steam can be generated.

[0061] In the case of the low-emission hydrocarbon cracking system 500, the cracked effluent gas 8 may pass through each of the second steam effluent exchanger 62, a transfer line exchanger 32 that is downstream of the second steam effluent exchanger 62, and the first steam effluent exchanger 61 that is downstream of the transfer line exchanger 32. As in previous examples, boiler feed water 3 can be sent to a steam drum 33, and boiler water 9a from the steam drum 33 may be directed to the transfer line exchanger 32. The boiler water 9a can be at least partly vaporized in the transfer line exchanger 32 by heat recovered from the stream of cracked effluent gas 8 passing therethrough, and the partly vaporized boiler water 9b can then flow back to the steam drum 33 by natural circulation. In the steam drum 33, water can be separated from high pressure steam 4 and the high pressure steam 4 can be sent to the first steam effluent exchanger 61 where the high pressure steam 4 can be initially superheated in a first stage superheating process using heat recovered from the stream of cracked effluent gas 8 passing through the first steam effluent exchanger 61. The high pressure steam 4 initially superheated in the first stage superheating process can then be subjected to second stage superheating by directing the superheated high pressure steam 4 from the first steam effluent exchanger 61 to the second steam effluent exchanger 62 and using the heat in the stream of cracked effluent gas 8 passing through the second steam effluent exchanger 62 to further superheat the superheated high pressure steam 4.

[0062] The flow of the high pressure steam 4 through the first steam effluent exchanger 61 and the second steam effluent exchanger 62 is counter-current to the flow of cracked effluent gas 8 in this example. The temperature of the superheated high pressure steam 4 leaving the second steam effluent exchanger 62 may be controlled by passing the superheated high pressure steam through a desuperheater 34 prior to its introduction to the second steam effluent exchanger 62. The desuperheater 34 may operate as described above for this purpose.

[0063] As may be understood from FIG. 5 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 501 of the low-emission hydrocarbon cracking system 500 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 501. Thus, the non-process duty load of the convection section 20 of the cracking furnace 501 is again minimized, and is also further reduced relative to the previous low-emission hydrocarbon cracking system examples presented herein, as use of the second steam effluent exchanger 62 for second stage superheating of the high pressure steam 4 eliminates the need to provide the convection section 20 of the cracking furnace 501 with the high pressure steam superheater 25 of the previous examples.

[0064] FIG. 6 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 600 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 601 the low-emission hydrocarbon cracking system 600. The cracking furnace 601 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 600 is also shown to include a cooling apparatus 602 including a transfer line exchanger 32 and a first and a second steam effluent exchanger 61, 62 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 601 can be cooled and by which superheated high pressure steam can be generated.

[0065] Operation of the low-emission hydrocarbon cracking system 600 is substantially the same as described relative to the low-emission hydrocarbon cracking system 500, except that the flow of the high pressure steam 4 through the first steam effluent exchanger 61 is co-current to the flow of cracked effluent gas 8 through the first steam effluent exchanger 61, while the flow of the superheated high pressure steam 4 through the second steam effluent exchanger 62 remains counter-current to the flow of cracked effluent gas 8 therethrough as in the low-emission hydrocarbon cracking system 500. A desuperheater 34 may again be used to control the temperature of the superheated high pressure steam 4 leaving the second steam effluent exchanger 62.

[0066] As may be understood from FIG. 6 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 601 of the low-emission hydrocarbon cracking system 600 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 601. Thus, the non-process duty load of the convection section 20 of the cracking furnace 601 is again minimized, and is also further reduced relative to the previous low-emission hydrocarbon cracking system examples presented in FIGS. 1-4, as use of the second steam effluent exchanger 62 for second stage superheating of the high pressure steam 4 eliminates the need to provide the convection section 20 of the cracking furnace 601 with the high pressure steam superheater 25 of the previous examples.

[0067] FIG. 7 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 700 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 701 of the low-emission hydrocarbon cracking system 700. The cracking furnace 701 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 700 is also shown to include a cooling apparatus 702 including a transfer line exchanger 32 and a first and a second steam effluent exchanger 61, 62 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 701 can be cooled and by which superheated high pressure steam can be generated.

[0068] In the case of the low-emission hydrocarbon cracking system 700, the cracked effluent gas 8 may pass through each of the second steam effluent exchanger 62, the first steam effluent exchanger 61 that is downstream of the second steam effluent exchanger 62, and the transfer line exchanger 32 that is downstream of the first steam effluent exchanger 61. As in previous examples, boiler feed water 3 can be sent to a steam drum 33, and boiler water 9a may be supplied by the steam drum 33 and directed to the transfer line exchanger 32. The boiler water 9a can be at least partly vaporized in the transfer line exchanger 32 by heat recovered from the stream of cracked effluent gas 8 passing therethrough, and the partly vaporized boiler water 9b can then flow back to the steam drum 33 by natural circulation. In the steam drum 33, water can be separated from high pressure steam 4 and the high pressure steam 4 can be sent to the first steam effluent exchanger 61 where the high pressure steam 4 can be initially superheated in a first stage superheating process using heat recovered from the stream of cracked effluent gas 8 passing through the first steam effluent exchanger 61. The high pressure steam 4 initially superheated in the first stage superheating process can then be subjected to second stage superheating by directing the superheated high pressure steam 4 from the first steam effluent exchanger 61 to the second steam effluent exchanger 62 and using the waste heat in the stream of cracked effluent gas 8 passing through the second steam effluent exchanger 62 to further superheat the high pressure steam 4. A desuperheater 34 may again be used to control the temperature of the superheated high pressure steam 4 leaving the second steam effluent exchanger 62. As may be observed in FIG. 7, the flow of the high pressure steam 4 through the first steam effluent exchanger 61 and the flow of the superheated high pressure steam 4 through the second steam effluent exchanger 62 is counter-current to the flow of cracked effluent gas 8 in this example.

[0069] As may be understood from FIG. 7 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 701 of the low-emission hydrocarbon cracking system 700 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 701. Thus, the non-process duty load of the convection section 20 of the cracking furnace 701 is again minimized, and is also further reduced relative to the previous low-emission hydrocarbon cracking system examples presented in FIGS. 1-4, as use of the second steam effluent exchanger 62 for second stage superheating of the high pressure steam 4 eliminates the need to provide the convection section 20 of the cracking furnace 701 with the high pressure steam superheater 25 of the previous examples.

[0070] FIG. 8 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 800 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 801 the low-emission hydrocarbon cracking system 800. The cracking furnace 801 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 800 is also shown to include a cooling apparatus 802 including a transfer line exchanger 32 and a first and a second steam effluent exchanger 61, 62 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 801 can be cooled and by which superheated high pressure steam can be generated.

[0071] Operation of the low-emission hydrocarbon cracking system 800 is substantially the same as described relative to the low-emission hydrocarbon cracking system 700, except that the flow of the high pressure steam 4 through the first steam effluent exchanger 61 is co-current with the flow of cracked effluent gas 8 therethrough, while the flow of the superheated high pressure steam 4 through the second steam effluent exchanger 62 remains counter-current as in the low-emission hydrocarbon cracking system 700. A desuperheater 34 may again be used to control the temperature of the superheated high pressure steam 4 leaving the second steam effluent exchanger 62.

[0072] As may be understood from FIG. 8 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 801 of the low-emission hydrocarbon cracking system 800 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 801. Thus, the non-process duty load of the convection section 20 of the cracking furnace 801 is again minimized, and is also further reduced relative to the previous low-emission hydrocarbon cracking system examples presented in FIGS. 1-4, as use of the second steam effluent exchanger 62 for second stage superheating of the high pressure steam 4 eliminates the need to provide the convection section 20 of the cracking furnace 801 with the high pressure steam superheater 25 of the previous examples.

[0073] FIG. 9 is a schematic representation of another example configuration of a low-emission hydrocarbon cracking system 900 in which high pressure steam is superheated at least in part by a mechanism other than a convection section 20 of a cracking furnace 901 the low-emission hydrocarbon cracking system 900. The cracking furnace 901 can be seen to again include a radiant section 10 and a convection section 20. The low-emission hydrocarbon cracking system 900 is also shown to include a cooling apparatus 902 including a transfer line exchanger 32 and a first and a second steam effluent exchanger 61, 62 via which a stream of hot cracked effluent gas 8 discharged from the cracking furnace 901 can be cooled and by which superheated high pressure steam can be generated.

[0074] Operation of the low-emission hydrocarbon cracking system 900 is substantially the same as described relative to the low-emission hydrocarbon cracking system 700, except that the flow of the high pressure steam 4 through the first steam effluent exchanger 61 remains counter-current to the flow of cracked effluent gas 8 therethrough as in the low-emission hydrocarbon cracking system 700, while the flow of the superheated high pressure steam 4 through the second steam effluent exchanger 62 is co-current with the flow of cracked effluent gas 8 therethrough. A desuperheater 34 may again be used to control the temperature of the superheated high pressure steam 4 leaving the second steam effluent exchanger 62.

[0075] As may be understood from FIG. 9 and the above description, at least partially superheating the high pressure steam 4 outside of the convection section 20 of the cracking furnace 901 of the low-emission hydrocarbon cracking system 900 removes what would otherwise be additional duty from the convection section 20 of the cracking furnace 901. Thus, the non-process duty load of the convection section 20 of the cracking furnace 901 is again minimized, and is also further reduced relative to the previous low-emission hydrocarbon cracking system examples presented in FIGS. 1-4, as use of the second steam effluent exchanger 62 for second stage superheating of the high pressure steam 4 eliminates the need to provide the convection section 20 of the cracking furnace 901 with the high pressure steam superheater 25 of the previous examples.

[0076] FIG. 10 is a schematic representation of a portion of another example configuration of a low-emission hydrocarbon cracking system in which high pressure steam is superheated at least in part by a mechanism other than a convection section of a cracking furnace of the low-emission hydrocarbon cracking system. Operation of the low-emission hydrocarbon cracking system represented in FIG. 10 is substantially the same as described relative to the low-emission hydrocarbon cracking system 100 of FIG. 1, except that the flow of the high pressure steam 4 exiting the steam effluent exchanger 60 of a cooling apparatus of the low-emission hydrocarbon cracking system is not subjected to second stage superheating in the convection section 20 of the cracking furnace, nor elsewhere. The flow of high pressure steam 4 through the steam effluent exchanger 60 is counter-current to the flow of cracked effluent gas 8 through the steam effluent exchanger 60 in this example.

[0077] FIG. 11 is a schematic representation of a portion of another example configuration of a low-emission hydrocarbon cracking system in which high pressure steam is superheated at least in part by a mechanism other than a convection section of a cracking furnace of the low-emission hydrocarbon cracking system. Operation of the low-emission hydrocarbon cracking system represented in FIG. 11 is substantially the same as described relative to the low-emission hydrocarbon cracking system 200 of FIG. 2, except that the flow of the high pressure steam 4 exiting the steam effluent exchanger 60 of a cooling apparatus of the low-emission hydrocarbon cracking system is not subjected to second stage superheating in the convection section 20 of the cracking furnace, nor elsewhere. The flow of high pressure steam 4 through the steam effluent exchanger 60 is co-current to the flow of cracked effluent gas 8 through the steam effluent exchanger 60 in this example.

[0078] FIG. 12 is a schematic representation of a portion of another example configuration of a low-emission hydrocarbon cracking system in which high pressure steam is superheated at least in part by a mechanism other than a convection section of a furnace of the low-emission hydrocarbon cracking system. Operation of the low-emission hydrocarbon cracking system represented in FIG. 12 is substantially the same as described relative to the low-emission hydrocarbon cracking system 300 of FIG. 3, except that the flow of the high pressure steam 4 exiting a steam effluent exchanger 60 of a cooling apparatus of the low-emission hydrocarbon cracking system is not subjected to second stage superheating in the convection section 20 of the cracking furnace, nor elsewhere. The flow of high pressure steam 4 through the steam effluent exchanger 60 is counter-current to the flow of cracked effluent gas 8 through the steam effluent exchanger 60 in this example.

[0079] It should be noted that in the case of each of the low-emission hydrocarbon cracking system examples presented in FIGS. 1-12, the process duty of the convection section 20 of the associated cracking furnace is unaffected by the various configurations via which the superheating of the high pressure steam 4 can be performed partially or entirely outside of the convection section 20 of the cracking furnace. Consequently, further efficiency increases to the radiant sections of the cracking furnaces of the example low-emission hydrocarbon cracking systems can be made while still allowing the low-emission hydrocarbon cracking systems to process hydrocarbon feedstocks having boiling curves with a high end point, such as condensates, gas oils, vacuum gas oils (VGO), hydrotreated vacuum gas oils (HVGO) and plastic-based oils, without a fear of fouling on the shell side of the exchanger, which is contrary to the feed effluent exchanger design of known cracking systems.

[0080] Further, the metal surface temperature of the steam effluent exchangers employed in example configurations of the low-emission hydrocarbon cracking systems according to the disclosure can be substantially higher than that of a transfer line exchanger associated with a traditional cracking furnace. As such, condensation and subsequential fouling of the heavy ends of the components in the cracked effluent gas where the dew point of the components exceeds the surface temperature of a steam effluent exchanger can be either reduced or prevented. This can be emphasized by providing a co-current high pressure steam flow path through the steam effluent exchanger, which may raise the surface temperature of the steam effluent exchanger above the dew point of the cracked effluent gas, and may be an especially effective technique at a location where the cracked effluent gas temperature is the coldest.

[0081] It should be understood that the example low-emission hydrocarbon cracking system configurations 500, 600, 700, 800, 900 illustrated in FIGS. 5-9 minimize the duty of the convection sections 20 of the cracking furnaces thereof to a greater extent than the example low-emission hydrocarbon cracking system configurations 100, 200, 300, 400 illustrated in FIGS. 1-4 because the example low-emission hydrocarbon cracking systems 500, 600, 700, 800, 900 are able to adequately superheat the high pressure steam entirely outside of the convection sections 20 of their respective cracking furnaces. Therefore, the example low-emission hydrocarbon cracking system configurations 500, 600, 700, 800, 900 also have the highest potential for reducing cracking furnace fuel consumption and associated CO 2 emissions by permitting greater efficiency improvements to the radiant section 10 of the cracking furnace. It has been determined, for example, that use of an example low-emission hydrocarbon cracking system 500, 600, 700, 800, 900 can reduce fuel consumption and CO 2 emissions by approximately 25% in comparison with a cracking furnace wherein high pressure steam superheating is performed in the convection section thereof.

[0082] In some examples, the arrangement of a steam effluent exchanger relative to a transfer line exchanger in the cracked effluent gas flow path can facilitate the use of a smaller steam effluent exchanger. For example, locating a steam effluent exchanger upstream of a transfer line exchanger instead of downstream of a transfer line exchanger may allow for a reduction in the required surface area of the steam effluent exchanger. This may be the case because when the steam effluent exchanger is upstream of the transfer line exchanger, the steam effluent exchanger receives the cracked effluent gas at a hotter temperature than would be the case in an opposite arrangement, and a lesser steam effluent exchanger surface area is resultantly required to adequately superheat the high pressure steam. The same can hold true relative to an arrangement of multiple steam effluent exchangers.

[0083] In some examples, the selection of a co-current or counter-current flow of the high pressure steam through a steam effluent exchanger can permit different steam effluent exchanger constructions. For example, the inlet area of a steam effluent exchanger is typically the hottest part of the steam effluent exchanger because the temperature of cracked effluent gas is hotter at the inlet than at the outlet or elsewhere within the steam effluent exchanger. The high temperature of the cracked effluent gas can push the functional limits of the material from which the steam effluent exchanger is constructed. At least in such a case, employing a co-current steam effluent exchanger design can reduce the maximum steam effluent exchanger tube wall temperature and, in turn, the required tube wall thickness. Consequently, steam effluent exchanger materials may be maintained within a safe operating range, or construction materials that may not otherwise be usable may be usable, by employing a co-current steam effluent exchanger design.

[0084] FIG. 13 is a flowchart 1000 describing one example of a method of operating a low-emission hydrocarbon cracking system to convert a hydrocarbon feedstock into a cracked effluent gas. As indicated in block 1002 a hydrocarbon feedstock can be preheated in a convection section of a cracking furnace of the low-emission hydrocarbon cracking system. The convection section may be positioned to receive a flow of flue gas generated by a combustion of fuel in a radiant section of the cracking furnace. The convection section may include a number of convection banks that can be utilized for various purposes, such as to perform other process or non-process (convection) heating operations. The heating operations for which the convection section is responsible defines the required duty capacity of the convection section.

[0085] As indicated in block 1004 the cracked effluent gas can be generated by heating the preheated hydrocarbon feedstock in at least one radiant coil arranged in the radiant section of the cracking furnace. In some examples, the cracked effluent gas may be produced by heating the preheated hydrocarbon feedstock in a radiant section of the cracking furnace such that the preheated hydrocarbon feedstock is exposed to sufficient combustion heat to cause a pyrolysis of the hydrocarbon feedstock. In other examples, the heat required to crack the hydrocarbon feedstock may be produced in the radiant section by electric heating elements instead of by fuel combustion.

[0086] As indicated in block 1006 the cracked effluent gas may be cooled upon exiting the radiant section of the cracking furnace. Cooling of the cracked effluent gas leaving the radiant section may be performed, for example, to stop pyrolysis side reactions, and to preserve the equilibrium of the reactions in favor of the products generated during pyrolysis. In some examples, cooling of the cracked effluent gas can be accomplished, at least in part, using a transfer line exchanger.

[0087] As indicated in block 1008, at least partly vaporized boiler water can be generated by heating boiler water in the transfer line exchanger using waste heat from the cracked effluent gas. Vaporizing the boiler water using waste heat from the cracked effluent gas simultaneously cools the cracked effluent gas. The boiler water may be supplied from, for example, a steam drum that is in fluid communication with the transfer line exchanger. When present, the steam drum may also receive a flow of boiler feed water to replenish the boiler water supply. As indicated in block 1010, high pressure steam can be generated, at least in part, from the at least partly vaporized boiler water generated by the transfer line exchanger.

[0088] As indicated in block 1012, superheated high pressure steam can be generated by heating a flow of the high pressure steam in a steam effluent exchanger using waste heat from the cracked effluent gas. The steam effluent exchanger may be placed in a flow path of the cracked effluent gas, such that the hot cracked effluent gas passes through the steam effluent exchanger. The steam effluent exchanger may be arranged in series with the transfer line exchanger. The steam effluent exchanger may be located upstream or downstream from the transfer line exchanger. When multiple steam effluent exchangers are used, one or more steam effluent exchangers may be located upstream of the transfer line exchanger and one or more steam effluent exchangers may be located downstream of the transfer line exchanger. In another example, all of the multiple steam effluent exchangers can be located upstream of the transfer line exchanger. The flow of the high pressure steam through a steam effluent exchanger may be co-current with or counter-current to the flow of the cracked effluent gas through the steam effluent exchanger. When multiple steam effluent exchangers are used, the flow of the high pressure steam through each of the steam effluent exchangers may be co-current with, counter-current to, or some combination of co-current with and counter-current to, the flow of the cracked effluent gas through the steam effluent exchangers.

[0089] While the various examples of low-emission hydrocarbon cracking systems presented herein all include a radiant section that relies on the combustion of a fuel to generate the heat necessary to crack the hydrocarbon feedstock, it should be understood that the cracking heat may be generated electrically in other examples. That is, the low-emission cracking furnace examples may be electric cracking furnaces. In still other examples, cracking of the hydrocarbon feedstock may be performed using a rotodynamic device, which is a device capable of causing the pyrolysis reaction of the hydrocarbon feedstock using kinetic energy generated via static and dynamic rotors. The use of other radiant section designs may also be possible.

[0090] The foregoing description of certain examples, including illustrated examples, has been presented only for the purpose of illustration and description and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Numerous modifications, adaptations, and uses thereof will be apparent to those skilled in the art without departing from the scope of the disclosure.

Examples

Embodiment Construction

[0024]Certain aspects and examples of the present disclosure relate to a low-emission hydrocarbon cracking system that includes a cracking furnace having a convection section and a radiant section and is operable to crack a hydrocarbon feedstock. The low-emission hydrocarbon cracking system may achieve lower emissions by, for example, facilitating the ability to increase the efficiency of the radiant section (e.g., firebox) of the cracking furnace without exceeding a duty capacity of a convection section of the cracking furnace. This may be achieved in some examples by utilizing configurations where the process of superheating high pressure steam used for power generation or other downstream processes is performed partly or completely outside the convection section of the cracking furnace. Superheating the high pressure steam outside the convection section lessens the heat demand of the convection section on the flue gas that is generated during the hydrocarbon cracking process. Thi...

Claims

1. A hydrocarbon cracking system for converting a hydrocarbon feedstock into a cracked effluent gas comprising: furnace comprising: a convection section comprising at least one hydrocarbon feedstock preheater; a radiant section comprising at least one radiant coil arranged to receive the preheated hydrocarbon feedstock and to generate the cracked effluent gas therefrom; and a cooling apparatus arranged to receive the cracked effluent gas, the cooling apparatus comprising: a transfer line exchanger arranged to receive a boiler water flow and to generate at least partly vaporized boiler water therefrom using waste heat from the cracked effluent gas; and a steam effluent exchanger arranged to receive a high pressure steam flow and to generate superheated high pressure steam therefrom using waste heat from the cracked effluent gas.

2. The hydrocarbon cracking system of claim 1, further comprising a high pressure steam source that receives the at least partly vaporized boiler water from the transfer line exchanger, the high pressure steam source being in fluid communication with the steam effluent exchanger.

3. The hydrocarbon cracking system of claim 2, wherein: the steam effluent exchanger is arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas; the steam effluent exchanger is located either downstream or upstream of the transfer line exchanger; and a flow path of the high pressure steam flow through the steam effluent exchanger is either counter-current to or co-current with the flow path of the cracked effluent gas.

4. The hydrocarbon cracking system of claim 2, further comprising a high pressure steam superheater located in the convection section of the furnace and arranged to receive the superheated high pressure steam from the steam effluent exchanger and to further superheat the superheated high pressure steam.

5. The hydrocarbon cracking system of claim 2, wherein: the steam effluent exchanger comprises at least two steam effluent exchangers; wherein a first steam effluent exchanger of the at least two steam effluent exchangers is arranged to receive the high pressure steam flow from the high pressure steam source and to generate the superheated high pressure steam therefrom; wherein a second steam effluent exchanger of the at least two steam effluent exchangers is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam; and a desuperheater is located in a flow path of the superheated high pressure steam between the first steam effluent exchanger and the second steam effluent exchanger and is operable to control a temperature of the superheated high pressure steam leaving the second steam effluent exchanger by injecting cold boiler feed water into the superheated high pressure steam.

6. The hydrocarbon cracking system of claim 2, wherein: the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas; the first steam effluent exchanger is located downstream of the transfer line exchanger and the second steam effluent exchanger is located upstream of the transfer line exchanger along the flow path of the cracked effluent gas; the first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas; and the second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

7. The hydrocarbon cracking system of claim 2, wherein: the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas; the first steam effluent exchanger is located downstream of the transfer line exchanger and the second steam effluent exchanger is located upstream of the transfer line exchanger along the flow path of the cracked effluent gas; the first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is co-current with the flow path of the cracked effluent gas; and the second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

8. The hydrocarbon cracking system of claim 2, wherein: the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas; the first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas; the first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas; and the second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

9. The hydrocarbon cracking system of claim 2, wherein: the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with the transfer line exchanger in a flow path of the cracked effluent gas; the first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas; the first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is co-current with the flow path of the cracked effluent gas; and the second steam effluent exchanger is arranged to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas.

10. The hydrocarbon cracking system of claim 2, wherein: the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger in series with a transfer line exchanger in a flow path of the cracked effluent gas; the first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas; the first steam effluent exchanger is arranged to receive the high pressure steam flow from the high pressure steam source and to generate superheated high pressure steam by directing the high pressure steam flow through the first steam effluent exchanger along a flow path that is counter-current to the flow path of the cracked effluent gas; and the second steam effluent exchanger is configured to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and to further superheat the superheated high pressure steam by directing the flow of the superheated high pressure steam through the second steam effluent exchanger along a flow path that is co-current with the flow path of the cracked effluent gas.

11. A heat exchanger and cracking furnace arrangement comprising: a transfer line exchanger positioned in a cracked effluent gas flow path from a cracking furnace; and a steam effluent exchanger positioned in the cracked effluent gas flow path in series with the transfer line exchanger; wherein the transfer line exchanger comprises a boiler water input and an at least partly vaporized boiler water output, the transfer line exchanger configured to at least partly vaporize the boiler water using waste heat from the cracked effluent gas; and wherein the steam effluent exchanger comprises a high pressure steam input and a superheated high pressure steam output, the steam effluent exchanger configured to generate the superheated high pressure steam using waste heat from the cracked effluent gas.

12. The arrangement of claim 11, wherein the steam effluent exchanger comprises at least two steam effluent exchangers; a first steam effluent exchanger of the at least two steam effluent exchangers is arranged to receive a high pressure steam flow from a high pressure steam source and to generate the superheated high pressure steam therefrom; a second steam effluent exchanger of the at least two steam effluent exchangers is positioned to receive a flow of the superheated high pressure steam from the first steam effluent exchanger and configured to further superheat the superheated high pressure steam; and wherein the second steam effluent exchanger is configured to direct the flow of the superheated high pressure steam through the second steam effluent exchanger along a path that is co-current with or counter-current to a path of the high pressure steam flow through the first steam effluent exchanger optionally, further comprising a desuperheater positioned in a flow path of the superheated high pressure steam between the first steam effluent exchanger and the second steam effluent exchanger and operable to control a temperature of the superheated high pressure steam leaving the second steam effluent exchanger by injecting cold boiler feed water into the superheated high pressure steam.

13. A method of operating a hydrocarbon cracking system to convert a hydrocarbon feedstock into a cracked effluent gas comprising: preheating the hydrocarbon feedstock in a convection section of a cracking furnace of the hydrocarbon cracking system; generating a cracked effluent gas by heating the preheated hydrocarbon feedstock in a radiant section of the cracking furnace; cooling the cracked effluent gas by passing the cracked effluent gas through a transfer line exchanger; generating at least partly vaporized boiler water by heating boiler water in the transfer line exchanger using waste heat from the cracked effluent gas; generating high pressure steam, at least in part, from the at least partly vaporized boiler water; and generating superheated high pressure steam by heating a flow of the high pressure steam in a steam effluent exchanger using waste heat from the cracked effluent gas; optionally, wherein: the steam effluent exchanger is arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas; the steam effluent exchanger is located either downstream or upstream of the transfer line exchanger along the flow path of the cracked effluent gas; and the high pressure steam flows through the steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas; or wherein the superheated high pressure steam generated in the steam effluent exchanger is further superheated by directing the superheated high pressure steam to a high pressure steam superheater located in the convection section of the cracking furnace.

14. The method of claim 13, wherein: the high pressure steam is provided by a steam drum from a combination of the boiler water present in the steam drum and the at least partly vaporized boiler water generated in the transfer line exchanger; and the flow of the high pressure steam is received by the steam effluent exchanger from the steam drum.

15. The method of claim 13, wherein : the steam effluent exchanger comprises at least two steam effluent exchangers; a first steam effluent exchanger of the at least two steam effluent exchangers receives the flow of the high pressure steam from a high pressure steam source and generates the superheated high pressure steam therefrom; a second steam effluent exchanger of the at least two steam effluent exchangers receives a flow of the superheated high pressure steam from the first steam effluent exchanger and further superheats the superheated high pressure steam; and a desuperheater is located in a flow path of the superheated high pressure steam between the first steam effluent exchanger and the second steam effluent exchanger and is used to control a temperature of the superheated high pressure steam leaving the second steam effluent exchanger by injecting cold boiler feed water into the superheated high pressure steam; or wherein: the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger, and the first steam effluent exchanger and the second steam effluent exchanger are arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas; the first steam effluent exchanger is located downstream of the transfer line exchanger along the flow path of the cracked effluent gas and the second steam effluent exchanger is located upstream of the transfer line exchanger along the flow path of the cracked effluent gas; the superheated high pressure steam is generated in the first steam effluent exchanger by directing the flow of the high pressure steam through the first steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas; and the superheated high pressure steam generated in the first steam effluent exchanger is further superheated in the second steam effluent exchanger by directing a flow of the superheated high pressure steam through the second steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas; or wherein: the steam effluent exchanger comprises a first steam effluent exchanger and a second steam effluent exchanger, and the first steam effluent exchanger and the second steam effluent exchanger are arranged in series with the transfer line exchanger in a flow path of the cracked effluent gas; the first steam effluent exchanger and the second steam effluent exchanger are both located upstream of the transfer line exchanger along the flow path of the cracked effluent gas, and the second steam effluent exchanger is located upstream of the first steam effluent exchanger along the flow path of the cracked effluent gas; the superheated high pressure steam is generated in the first steam effluent exchanger by directing the flow of the high pressure steam through the first steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas; and the superheated high pressure steam generated in the first steam effluent exchanger is further superheated in the second steam effluent exchanger by directing a flow of the superheated high pressure steam through the second steam effluent exchanger in a direction that is either counter-current to or co-current with the flow path of the cracked effluent gas.

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